5. Through the Bottleneck
5-1. A Most Peculiar Mammal: Homo sapiens
5-2. Genetic Bottleneck
5-3. Before the Bottleneck
5-4. After the Bottleneck
5-5. Winner Takes All
In this chapter we are entering a much more controversial territory.
It is in the nature of this fascinating but difficult subject that important finds are rare and gaps in our knowledge prominent. The limited evidence is always inadequate for unambigious answers and the same evidence can be and is interpreted in widely different ways by different specialists. Ferocious and n entertaining controversies result which, sometimes, bring progress. Science does not claim to "know everything", science is a method to collect and interpret and refine the evidence through dispute and controversy. This is nowhere more true than in palaoanthropology.
It is only fair to mention here that there are some palaeoanthropologists who do not think that there has been a bottleneck of Homo sapiens in the past, or that it has taken place much further back in time than we assume here, or that the impact of Toba was altogether negligible. On balance, we do not think that they are right,. Those readers who wish to inquire further into these controversial matters are referred to a number of papers that give alternative views (ref. Gathorne-Hardy F.J. et al., 2003, with response ref. Ambrose S.H. 2003; also ref. Hawks J. et al., 2000) and into the vast general literature available.
Opposition has come from supporters of the "regional continuity" (or "multiregional") theory of human evolution who do not agree with the various forms of the "Out Of Africa" theory. The multi-regional model has been sickly for some time and after the recent discovery of a sub-pygmoid Homo floresiensis (a dwarf-form of Homo erectus.
Necessarily, the discovery of Homo floresiensis bears on the debate over the origins of modern humans - whether Homo sapiens evolved in various regions throughout the world from Homo erectus populations, or as a distinct and recent African species. Multiregional evolution requires the existence of large populations for long periods, with isolation being rare or absent so that global species could evolve in a single direction. Palaeoanthropological and genetic studies have already done much to discredit this model, and Homo floresiensis puts yet another (the last?) nail in the multiregional coffin. Not only did Homo floresiensis evolve in the absence of gene exchange with other hominins, but no one can argue that LB1 (the first Homo floresiensis skeleton found) contributed to our own species' genetic make-up. (ref. Lahr M.M. et al., 2004)
While it is generally accepted today that Homo sapiens developed out of some kind of Homo erectus, it far from clear when, where, why and from what group of Homo erectus our earliest ancestors evolved from. There is a rather vague consensus that Homo sapiens started to develop out of some Homo erectus group around 160,000 years ago somewhere in Africa. What is clear is that some Homo erectus groups co-existed with Homo sapiens, even post-Toba. Homo erectus javaensis ("Java Man") lived on Java as recently as 25,000 to 30,000 years ago (ref. Swisher III C.C. et al. 2000) and Homo floresiensis on the Indonesian island of Flores survived even longer until around 18,000 years ago (ref. Lahr M.M. et al., 2004; Brown P. et al., 2004; Morwood M.R., et al., 2004). For more information, see sub-chapter 5-4 below.
5-1. A most peculiar mammal: Homo sapiens
Human beings in civilizations that think about such things, pride themselves on their (occasionally) towering intelligence, their scientific, technological, philosophical, economic and artistic accomplishments. Scientific nomenclature knows humans as Homo sapiens ("wise man"), nicely reflecting such high self-esteem.
Leaving the towering but hard-to-measure intelligence aside, there are other, much more easily measured if rather less well-publicised aspects of Homo sapiens that set off the species quite spectacularly from other life forms:
Of all living things on earth weighing more than a few grams/ounces or measuring more than a few cm/inches,
(a) Homo sapiens is today the only truly world-wide species, living as it does in flat and rugged, in hot and cold, in dry and wet places, and practically everywhere in between. The species has even gained a recent foothold in Antarctica, and in large cities it has created its very own environment.
(b) Homo sapiens has by far the largest numbers of individuals (estimated 6,300,000,000 in 2003)
(c) Local variations (known as "races") also show extraordinarily low levels of within-and between-population genetic variation in comparison to the apes. This also supports an extremely recent origin for Homo sapiens (ref. Ferris et al., 1981; Ruvolo et al., 1993). Only around 10% of the limited human genetic variation is accounted for by differences between populations (ref. Lewontin, 1972; Relethford, 1995).
(d) Homo sapiens has very little genetic diversity despite its huge numbers.
The last point is the oddest - and the least widely known. It is also one of the arguments in favour of a relatively recent bottleneck (e.g. Toba) rather than one much longer ago (e.g. one proposed for 2 million years ago by the "regional continuity" supporters, ref. Hawks, et al, 2000). The low genetic diversity implies that the present teeming multitudes of human beings goes back to a small and relatively recent founding population.
Genetics has this to say on Homo sapiens:
... we have sequenced 10 kb of non-coding DNA in a region of low recombination at Xq13.3 from 70 humans representing all major language groups of the world. In addition, the same sequence has been determined from 30 chimpanzees, representing all major subspecies, as well as bonobos. Comparison to humans reveals an almost four-fold higher diversity and a three-fold greater age of the most recent common ancestor of the chimpanzee sequences. Phylogenetic analyses show the sequences from the different chimpanzee subspecies to be intermixed ... These data, as well as preliminary work in the other great apes, indicate that the human genome is unique in carrying extremely little nucleotide diversity. (ref. Kaessman H. et al, 2000)
While human overall genetic diversity is low, what diversity exists, is highest in Africa. This fact is one of the major arguments in favour of the "Out of Africa" theory of human origins:
... the gene pool in Africa contains more variation than elsewhere, and the genetic variation found outside of Africa represents only a subset of that found within the African continent. From a genetic perspective, all humans are therefore Africans, either residing in Africa or in recent exile. (ref. Pääbo S., 2001)
5-2. Genetic bottleneck
What brought about this remarkable state?
There must have been a "genetic bottleneck" - a fairly recent one (as such things go) and a severe one. Other ideas besides a bottleneck have been floated but none has been convincing or stood the test of time and closer scrutiny. It does indeed seems that the human race at one time suffered a spectacular reverse in its fortunes. Before the reverse, our ancestors must have had the same wide genetic variety that nearest living relatives, the apes, still have today.
What is a "genetic bottleneck"? It is simply the genetic signature of a serious reduction in the number of living members of a species in the past. There are many possible causes: a disease that rampages through a population, predation when one animal for some reason suddenly becomes such a successful hunter that prey species are seriously reduced in numbers (maybe leaving the over-successful hunters to face a bottleneck of their own when facing starvation), or it can be the result of dramatic climatic changes, etc. The ultimate bottleneck (not so much a "bottleneck" as a "closed bottle", really) is extinction.The branching off of a new species from a pre-existing older population can also be defined as a bottleneck.
Species normally develop over long periods of time and in that time they accumulate genetic variations in their population. If a substantial part of a population is killed, there is an inevitable loss of genetic diversity among the survivors. The smaller the surviving population that comes through a bottleneck, the smaller the diversity among the survivors. That is why it is fairly certain that Homo sapiens has gone through a rather severe bottleneck: the species has not yet had the time to restore its depleted diversity.
While a bottleneck can be identified in the genome of a populaton, it is difficult to determine its intensity and duration: a severe bottleneck cannot be differentiated from a longer, less severe one (ref. Relethford et al., 1994). Still more difficult is it to identify when a bottleneck has taken place. The latter is largely a question of searching through time for a possible cause of a bottleneck and then trying to determine whether the available evidence fits the suspected cause. This is largely what has been done with the Toba YTT event which actually fits surprisingly well and so has a relatively high probability of actually being true. There is no other possible cause in sight but of course, there cannot ever be absolute certainty - there never is in studies of the past.
Fig. 5-1. Schematic progress of the post-Toba bottleneck. We do not really know precisely what happened, where and when during the Toba bottleneck. The previous chapters have shown the volcanological and climatological sequence of events. What the effect these events had on the then living population of Homo sapiens is difficult even to guess. For example, we do not know with any degree of certainty how big the pre-existing human population was, to what extent it was affected, what the death rate was and where, when the climax of the bottleneck was reached and when and with what speed or where he eventual recovery started. Still less do we know how the other survivors of the genus Homo (see chapter 5-4 ) or the apes managed to get through thiswould have been something along these lines:
1. Rapidly shrinking population: famine, cold, disease, etc, neighbouring groups converging and merging in sheltered spots, perhaps fighting over very rapidly shrinking ressources; necessity forces adaptive cooperative skills within and between groups, new technical capabilities (new hunting-gathering skills, clothing, etc). This must have been balanced by a remarkable degree of adaptability and resourcefulness.
2. Climax of bottleneck: only small numbers left, aftershock may still kill isolated some groups but recently developed cooperative and new technical skills now allow groups to survive (if no more) in sheltered locations. Groups unable to adjust fast enough would be gone by now.
3. Surviving human population adapts and begins to recover: number of people increases again, skills and newly refined tools developed under the stress of survive-or-die are further developed and refined (a process that is still going on and has never stopped among humans - it is known as technology).
OIS5: Oxygen Isotope Stage 5: hot and humid (from 130,000 to 73,000 years ago)
OIS4: Oxygen Isotope Stage 4: cold and dry (from 73,000 to 63,000 years ago), in fact the coldest OIS for the past 110,000 years
OIS3: Oxygen Isotope Stage 3: warm but not quite as warm or humid as OIS5 (63,000 bis 45,000 years ago)
It was no coincidence that in the eastern Mediterranean at the change from OIS5 to OIS4, i.e. just after the Toba YTT event, the Afro-Asian biotic community was abruptly replaced by a palaeoarctic one, including the already cold-adapted Homo neanderthalensis (ref. Tchernov E. 1992a; Ambrose S.H. 1998).
Estimating how low the number of members of the species Homo sapiens could have been to account for today's uniformity involves a number of variables that are anything but clear-cut. It has been estimated that only 40-600 females (which translates into a total population of less than 3,000 persons; Harpending H.C. et al. 1993) came through the bottleneck. Another estimate arrived at 500-3,000 females (ref. Rogers A.R. 1993) and yet another at 1,000 to 4,300 individuals (Ayala F.J. 1996; Takahata N. at al. 1995). The highest estimate so far has 10,000 females of reproductive age as the minimum (ref. Ambrose S.H.. 1998). Even if the highest estimate is accepted, we are talking about the entire human race numbering no more than the population of a small country town today.
Such a small group could not have been widespread but had to live (and surviva Toba) in a relatively small area. This was most likely in East Africa (ref. Jones J.S. et al.1986). The hardest hit area then inhabited by Homo sapiens was the Middle East, including probably the Arabian peninsula. It is an open question whether any groups of Homo sapiens could and did survive there. During the long, warm and humid OIS3 period, Homo sapiens had spread into the path of Toba's "kill zone". In the Middle East ash falls and volcanic winter must have come very suddenly and with devastating impact. There was no time to adapt and develop new survival strategies or even just move out of harm's way. It is likely that Homo sapiens groups living in Asia were wiped out (along with their genetic diversity). The further south a group was when overtaken by the darkening sky, the larger its chance of survival. We have seen that the bottleneck could have reduced mankind to as few as 40 females, plus their menfolk and children. This would then have represented the entire human population living on planet earth. Surviving stragglers further to the north would not have lived long. Even if there were 10,000 survivors, the severity of the Toba bottleneck was undoubtedly extremely hard on Homo sapiens. It is likely that the long-term survivors were living along the eastern coast of Africa, possibly on the southern side of the equator where the impact of Toba was least.
Fig. 5-2. The scene of Homo sapiens exploits during the pre-Toba OIS 5 between 160,000 and 73,000 years ago. For a discussion of the information contained in this figure see the text below.
white dot: Site of the oldest known Homo sapiens (Homo sapiens idaltu, Ethiopia, 154.000 to 160,000 years old. (ref. White T.D. et al., 2003; Clark J.D., 2003). See chapter 5-3 below.
white X1: the Levant (site of Homo sapiens/Homo neanderthalensis interactions ca. 100,000, 73,000 and 63,000 years ago).
white X2: Toba survivor Homo erectus on Java, Indonesia
white X3: Toba survivor Homo floresiensis on Flores, Indonesia
white-bordered area without hatching: area into which Homo sapiens had expanded from around 100,000 years ago (ref. Stringer C. et al., 1988) until the Toba eruption (the limits of this expansion are highly uncertain and include only the Levant for sure. Whether Mesopotamia and the Arabian peninsula was also settled has not been proven yet but it remains a possibility.
red dot: Toba volcano.
red line: The Toba "kill zone" where ash fall was likely to have a lethal and almost instant impact. In India the thickness of the ash layer found today reaches from 3 to in places 6 m, in Malaysia is reached a maximum of 9 m (see chapter 3). Further west, the ashfalls would have thinned gradually, but dust and areosol clouds would still have blocked the sun and brought on a calamitious drop in temperature - the forerunner of volcanic winter.
The dispersion, today, of gorillas, chimpanzees and bonobos is shown in the map above. The distribution of our closest living African relatives at the time of the first human expansion and at the time of the Toba event is not known, but they never ventured beyond Africa. The Gorilla split off from the Hominid ancestral line around 5 million years and the Chimpanzees a little later (the Orang Utan did the same 15 million years ago) (ref. Jones S., 1992). There is no evidence that the three African apes do not still live - more or less - in the same areas that they occupied when the first Homo sapiens developed out of a Homo erectus population to the east of them. Indeed, several populations of Chimpanzees that today still live in mountainous pleistocene ice age refugia in western Uganda and eastern Congo, are reported to have mismatch distributions suggesting a release from a bottleneck event around 67,000 years ago. The same Chimpanzees also suffered another bottleneck with a release around 20,000 years ago (ref. Goldberg T.I., 1996). At that time Homo sapiens had already acquired almost world-wide distribution, large numbers and a sophisticated cold-climate technology and was no longer much affected.
Homo erectus had used stone tools for at least 2 million years (and probably wooden tools even longer, but they have not survived) (ref. Probst E., 1992) and when at some time before 160,000 years ago, a new species developed out of a Homo erectus population, the new arrival continued with and built upon ancient tool-making traditions. Gorillas do not use tools, while chimpanzees sometimes use simple sticks and stones to catch termites or help crack nuts etc. but they, too, do not make specific tools. This is a major characteristic that distinguishes apes from members of the genus "Homo the toolmaker".
We have noted above that Homo sapiens has a highly reduced genetic diversity compared with his ape cousins. Evidence has been found of a similar but much milder bottleneck event among the African apes. It is highly probable that this was caused by the same event that reduced Homo sapiens to a few survivors. The African apes and the early humans were all living in Africa with the humans somewhat more widely spread. The question arises: why was Homo sapiens hit so much harder by the bottleneck event?
Fig 5-2 above may hold the answer:
- Africa has the largest well-watered tropical landmass in the world.
- Africa's highest and most extensive mountain chains (including Mt. Kilimanjaro) run rougly north-south along the east coast.
- Today, all African apes live in or on the western side of those mountains on a mostly "horizontal" east-west axis. That the apes suffered so much less genetic diversity suggests that this was already their distribution when volcanic winter struck 73,000 years ago. If there were apes east of the mountains, they did not survive.
- Early Homo sapiens had left most traces of its development and existence along the eastern coast of Africa where the species had spread on a mostly "vertical" north-south axis between the mountains and the coast, especially in Kenya and Ethiopia.
- Homo sapiens' first attempt at expanding out of Africa had led it through the fertile and (then) animal-rich valley of the Nile to the mediterranean African coast and through the Sinai peninsula into western Asia. Unhappily, the geographical distribution of Homo sapiens 73,000 years ago ensured almost maximum possible exposure to the direct and indirect effects of Toba. The African apes were living in the shelter of their mountain chains and were (their genetic diversity tells us) much less affected. They suffered some losses in a bottleneck event (ref. .Goldberg T.I.; Rogers A.R. et al 1996) but despite their lack of clothing, fire, or technology, they were comfortably off compared to to what Homo sapiens had to go through on the other side of the mountains and in the Levant.
Crossing the Sinai peninsula into western Asia and (perhaps) the Bab el Mandeb from Africa to the Arab peninsula could also have caused local bottlenecks, as only a small part of the following Toba bottleneck.
Homo sapiens had many characteristic that helped the species to survive where none of the apes could have done so. The attitude and aggressive drive behind the human expansion into northern Africa and western Asia was totally unlike the placid way of the African apes.
Homo sapiens is known as the "naked ape": at some stage in our evolutionary journey, our ancestors shed their fur, something none of the apes have done. We can only speculate about the likely causes or its timing. Was Homo erectus already a naked ape, or did Homo sapiens lose his fur only after branching off? Did the fur go early when early Homo started to develop its similarly peculiar upright two-legged posture? On two long legs, eye-centred Homo could see more and further, move faster and have his hands free for making tools and carrying weapons. Both loss of fur and upright posture were adaptations to savannah life. Perhaps they are connected.
However obscure the reasons and the timing, sweating followed by rapid cooling was as dangerous to the naked ape then as it still is to you and me today. Clothing would have been the solution from a very early stage - its so much more flexible than a permanent fur! The first clothing could have been worn a million or more years ago by Homo erectus. No evidence has, of course, survived from that far back and the oldest definitive evidence appears very late, in ice age art dating from 35,000 years ago (ref. Probst E. 1992). The fact that some humans obviously managed to come through a volcanic winter 73,000 years ago, however, can be taken as good evidence that our ancestors already had some way of covering and keeping themselves warm at short notice.
Fire would also have been a major help in getting through the dark post-Toba centuries and milennia. None of the apes has learnt to control it, but the genus Homo had used it long before Homo sapiens appeared. The oldest known deliberately constructed fireplace has been found in Kenya, at Chesowanja, and and it has been dated to about 1.4 million years (ref. Probst E. 1992).
Whatever the details, having the concept of "clothes" and a technology to make fire when needed would have made it that much easier to take defensive measures in the face of sudden volcanic winter: put on more clothing and gather around the fire to discuss the crisis.
5-3. Before the Bottleneck
Sometime around 100,000 years ago, Homo sapiens expanded its hunting territories within and out of Africa (ref. Ambrose S.H., 1998). The exact outer limits reached during this early expansion are unknown but definitively included the Levant and perhaps Mesopotamia as well as the Arabian peninsula. In the Levant, Homo sapiens along with a warmth-loving Afro-Arabian biotic community was supported by a warming climate. The cold-adapted Homo neanderthalensis along with its associated palaearctic biotic community on the other hand suffered and was pushed out of the Levant towards the north. The Toba eruption reversed this picture: Homo sapiens abruptly disappeard from the Levant and was, again, replaced by Homo neanderthalensis.
The failure of early modern humans to survive in the Levant during the early last glacial implies that they were not yet physiologically and/or behaviorally well-adapted to cold climates and Palaearctic environments, or at least not as well-adapted as Neanderthals. The first dispersal thus apparently failed to permanently establish modern humans outside Africa (ref. Ambrose S.H.,1998, p. 630-631).
During the two million years before the Toba bottleneneck there lived several human-like hominid species with large brains, upright walk (bipedalism) and small teeth. They are marked red in fig. 5-3 below. Some lived at the same time, some were ancestor, resp. descendant species. Evidence is scarce and often controversial. The following figures give a rough idea of the multitude of Hominid species in the seven million years before Toba. In fact there were probably more since it is unlikely that all Hominid species have been discovered.
Fig. 5-3. Homo sapiens and its extinct relatives. (chart
adapted from Clark J.D. 2003).
x marks the oldest known member of the species: Homo sapiens idaltu from Ethiopia (ref. ref. White T.D. et al., 2003; Clark J.D., 2003).
Fig. 5-4. Some data on various Homo species. The chart is no on the latest state of knowledge (which illustrates nicely how rapidly the field of palaeoanthropology moves as new information becomes available). "Grab-all groupings" such as "archaic Homo sapiens" and "early modern Homo sapiens" are not widely used anymore. (chart from Jones. S. et al, 1992)
Homo sapiens idaltu
Fig. 5-5. Homo sapiens idaltu is the earliest known unambiguous Homo sapiens. The species lived in Ethiopia and has been dated to around 160,000 years.
It was Homo sapiens idaltu - or someone very much like him - who moved down the Nile valley and into northern Africa and western Asia.
Homo sapiens idaltu seems to have practiced postmortem defleshing with stone tools, a practice known from other and earlier African species. "Polishing and intentional scraping modifications evident on two of these crania indicate that (Homo sapiens idaltu) may have manipulated the crania of their dead in mortuary practices whose dimensions, context and meaning might be revealed only by further discoveries." (quote from White T.D. et al., 2003. Nature, 423:751). (photographs and drawings curtesy of Nature 423; refs. White T.D. et al., 2003; Stringer Ch., 2003)
The only direct evidence of how the Toba eruption must have devastated the life of people living at the time of the eruption has been found in Malaysia. A large and long-established "palaeolithic workshop" named Kota Tampan, had been found and first excavated in 1938 by the geologist Collings (ref. Collings H.D., 1938; Ninkovitch D., 1978; Tjia H.D., 1993). In 1987, Prof. Zuraina Majid, archaeologist at the Malaysian University surveyed the site and excavated several hundred meters away from the original Collings dig. She found more tools so that the total extent of the site (or sites) imust be very considerable.
First thought 35,000 years old because of contaminated C14 measurements on wood samples found under the ash, the ash has now been re-dated in 1978 ( ref. Ninkovich D. et al., 1978) with volcanological methods and unambiguousoly assigned to the Toba YTT event of 73,000 years ago.
Some 50,000 stone tools have now been found and a great many more are likely to be awaiting discovery. Some of these tools are very basic and it is not always easy to identify them as tools.
The workshop may have still been in production until just before the Toba eruption. It is most unfortunate that noremains of the people working the site have been found, most probably Homo erectus. Perhaps they had time to flee and (one assumes in view of the size of the eruption) perished shortly afterwards but elsewhere.
Fig. 5-6. (above and below) The palaeolithic site of
Kota Tampan in Malaysia.
1. adapüted from Ninkovich D., et al. 1978;
2. Photzograph copies from Collings H.D., 1938, scale in feet;
3. adapted from Collings H.D., 1938,
4. adapted from Tjia H.D., 1993
blue: prehistoric lake at the time of Kota Tampan and other prehistoric features
black and grey: modern river Parak (Perak) with dam and
(map adapted from ref. Tjia H.D., 1993)
Fig 5-7. That Homo neanderthalensis lived through and survived the Toba botttleneck and its aftermath is not surprising. Neanderthal man was by far the most cold-adapted of all Homo species. The stocky build and shorter stature of Neanderthal in comparison to Homo sapiens are adaptations to cold.
1 Homo neanderthalensis, 2 Homo sapiens
Skhul cave on Mount Carmel, Israel Quafzeh cave nearth Nazareth, Israel
(photograph curtesy Stringer C. at al., 1994)
(photograph curtesy Stringer C. at al., 1994)
Skhul cave on Mount Carmel, Israel
Quafzeh cave nearth Nazareth, Israel
Skhul 5 skull (Neanderthal or "early modern"), male, Quafzeh 9 skull (Neanderthal or "early modern"), probably female ,
the braincase is distinctly modern, the projection lower face and the bows
over the eyes are not
buried with the jaw bone of a wild boar in his arms
(photograph curtesy Coon C.S.,1968)
(photograph curtesy Coon C.S.,1968)
Skhul 5 skull (Neanderthal or "early modern"), male,
Quafzeh 9 skull (Neanderthal or "early modern"), probably female ,
the braincase is distinctly modern, the projection lower face and the bows
over the eyes are not
Reconstruction of the burial of Skhul 4 , a strongly-built, "early
modern" human. Quafzeh 11 skull of a 12-year old child; although remarkably modern
in appearance, it is with an estimated age of 120,000 years among the
oldest finds at the site.
(photograph curtesy Stringer C.S. et al., 1994)
(photograph curtesy Stringer C.S., et al., 1994)
Reconstruction of the burial of Skhul 4 , a strongly-built, "early
Quafzeh 11 skull of a 12-year old child; although remarkably modern
in appearance, it is with an estimated age of 120,000 years among the
oldest finds at the site.
Fig 5-8. In the Levant, Homo neanderthalensis re-occupied caves that they had occupied during colder early periods until around 130,000 years ago (OIS6) and then abandoned during the warm and humid OIS5 period to advancing Homo sapiens. Volcanic winter and the following bitterly cold OIS4 after 73,000 years ago saw Neanderthal returning to the Levant and re-occupy his ancient caves. When climatic conditions improved yet again with the onset of OIS3 around 60,000 years ago, Homo neanderthalensis once more retreated to the north - for the last time. Homo neanderthalensis is thought to have become extinct around 30,000 years ago. His last traces fade away on the Iberian peninsula in western Europe. This climatic and population see-saw is reflected in the finds of a number of caves in the Israeli Levant (e.g. Tabun, Skhul , Qafzeh, Amud, Kebara, and others).
Quafzeh has produced the largest sample of early modern Homo sapiens from anywhere covering a period between 80,000 and 120,000 years ago.
The archaeological details and classification of the finds at Qafzeh and Skhul as well as the other caves of the area are extremely complex and much of it is controversial. While we can dimly visualise the rough outlines of what happened there, the details are clearly vastly more complicated. Nevertheless, the climate-induced see-sawing between what possibly were several kinds of Homo neanderthalensis with what could have been several kinds of early Homo sapiens is discernible. (refs. Coon C.S., 1968; Tchernov E., 1992a and 1992b; Bar.Yosef O., 1994; Ambrose S.H., 1998; Stringer C. et al. 1994 ).
The overlapping and mixed Neanderthal-Human traits observed in a number of Levantine cave finds have been interpreted as evidence of mixing between Neanderthals and humans. Recent genetic research, however, has shown that Neanderthals and modern humans diverged 500,000 to 600,000 years ago, suggesting that though they may have lived at the same time, Neanderthals did not contribute genetic material to modern humans . The two are different species that would have been unable to cross-breed. A team of U.S. and German researchers (M. Stoneking and S. Pääbo) has extracted mitochondrial DNA from Neanderthal bone showing that the Neanderthal DNA sequence does indeed fall outside the normal variation of modern humans. (ref. Krings M.et al, 1997). How this evidence is to be reconciled is as yet an open question.
5-4. After the Bottleneck
For a long time it was thought that of all the the hominid groups, only Homo sapiens and Homo neanderthalensis had come through the bottleneck. The last decade has shown that this was not so. At least two Homo erectus species also made it, and more may await discovery.
In the following we have a look at each of the four groups that we know have come through:
While Homo neanderthalensis unquestionably did survive the Toba bottleneck, life was not easier for him. The following medical description of four Neanderthals from Shanidar cave gives a rare view of just how rough life was in those days (ref. Trinkaus E. et al, 1982):
Four of the adult Neanderthals from Shanidar Cave, Iraq, Shanidar 1, 3, 4, and 5, show evidence of antemortem trauma. Shanidar 1 sustained injuries to the right frontal squama, the left lateral orbit, the right humerus and right fifth metatarsal. Associated with this trauma are hypoplasia or atrophy of the right clavicle, scapula, and humerus, osteomyelitis of the right clavicle, degenerative joint disease at the right knee, ankle, and first tarsometatarsal joint, and remodeling of the left tibia. Shanidar 3 experienced trauma-related degenerative joint disease at the right talocrural and talocalcaneal joints and sustained a penetrating wound across the left ninth rib. Shanidar 4 suffered a fracture of the right seventh or eighth rib, and Shanidar 5 had a scalp wound over the left frontal. A high frequency of antemortem trauma associated with the survival of the injured individuals appears to have been characteristic of the Neanderthals.
Fig. 5-9. Shanidar cave in Iraq.
Pollen of wildflowers were found around the skull of
Shanidar 4 which has consequently been christened the "Flower Man". This led to
the entirely reasonable speculation that there had perhaps been a funeral
ceremony with flowers. It would be by far the oldest known. The assumption is
hard to prove since the pollen could also have been brought into the cave by
winds of small burrowing animals - but then, why should the pollen be
concentrated only around this man's head? The oddest thing about this particular
controversy is not the presence of pollen but the embittered ferocity with which
the dispute "flowers for the funeral or not" is being fought. Some
palaeontologists seem psychologically allergic to flower pollen, a problem that
clearly needs much more funds and further research.
Shanidar 4 is rather vaguely dated: to sometime between 70,000 and 33,000 years ago. (drawing from Stringer C. et al., 1994).
Neanderthal: "The Old Man from Shanidar" (Shanidar 1). This male shows evidence of multiple injuries
suffered long before death. This can only mean that he was cared for
during his lifetime by other members of his group. Neanderthal: "The Flower Man from Shanidar" (Shanidar 4).
Neanderthal: "The Old Man from Shanidar" (Shanidar 1).
This male shows evidence of multiple injuries suffered long before death. This can only mean that he was cared for during his lifetime by other members of his group.
Neanderthal: "The Flower Man from Shanidar" (Shanidar 4).
The cold 60,000 years between the Toba event until the beginning of the present warm Holocene 10,000 years ago should have been an ideal time for the expansion of cold-adapted Homo neanderthalensis. Instead, the species declined, withdrawing into ever-smaller refuge areas until it vanished sometime around 30,000 years ago in the Iberian peninsula.
On the other hand, Homo sapiens not only managed to survive volcanic winter and the following ice age despite being originally a warm-adapted creature. He positively boomed. Soon after the Toba event he can be found all over the old world, sometime around 60,000 years ago he appeared in Australia and sometime before 10,000 years ago (perhaps long before) he had reached the southern tip of South America. No other Hominid had ever even come close to an expansion like this.
It must have been the attitude towards technology that gave our ancestors the rapid flexibility and adaptability necessary for success even in most trying circumstances. Technology is the only discernible difference between the four survivors of the bottleneck. Our technology-savvy ancestors could adapt on a much shorter timescale than even he fastest biological evolutionary process could have provided. Neanderthal and and the two Homo erectus species to make it through the Toba bottleneck also had a certain amount of technology but it shows rather slow and ponderous development, if any, before they vanish into extinction. Whether Homo sapiens actively used his technological edge to push Neanderthal and others into extinction is unknown, but in view of the way Homo sapiens has treated members of his own species from antiquity to the present day, it is not totally unthinkable that he did.
Once started, Homo sapiens never stopped inventing new and improving existing technologies. In the very recent past, he has systematized his ever-growing and aggressive quest for new skills and knowledge in Science, making more knowledge and understanding possible along with ever more technological inventions, and in ever more rapid sequence. Only a few thousand years after the first cities appeared, while and others are heading out into insterstellar space.
Fig. 5-10. Waves of technological invention out of post-Toba Africa starting around 50,000 years ago - not coincidentally from the most likely area for Homo sapiens to have survived Toba in largest numbers. (ref. Ambrose S.H., 1998; Ambrose S.H., 1997; Bar-Yosef et al, 1996; Bischoff et al, 1994; Mercier et al, 1995; Goebel et al, 1993; Goebel et al, 1995).
Terminology differs between African and European archaeology: in Africa the main periods of stone tool technology are ESA, (Early Stone Age), MSA (Middle Stone Age) and LSA (Late Stone Age), while in Europe and Asia similar but later technologies are known as LP (Lower Palaeolithic), MP (Middle Palaeolithic) and UP (Upper Palaeolithic).
1 transformation MSA to LSA in east Africa, ca. 50,000 years ago
2 transformation MP to UP in the Levant, ca. 47,000-43,000 years ago
3 transformation MP to UP in western Europe, ca. 43,000-40,000 years ago
4 transformation MP to UP in Siberia, ca. 43,000-40,000 years ago
Homo neanderthalensis and Homo sapiens were not the only survivors of the genus Homo to come through the bottleneck. Two more have only recently been discovered and rightly caused a considerable stir.
It is no coincidence that both newly-discovered post-Toba survivors were found in Indonesia - upwind of the Toba YTT event and on the other side of the equator (see Fig. 5-2 ). Paradox as it may seem for areas so close to Toba to be among the least affected, it is nevertheless so. The earth rotates from west to east with the dominant air circulation moving from east to west - protecting areas to the east of Toba. The Orang Utan on Borneo (Kalimantan), the rich bio-diversity of Celebes (Sulawesi) and of Papua-New-Guinea (to give just three major examples) could have survived only because they were upwind from Toba 73,000 years ago and to have been on the southern side of the equator gave still more protection.
Homo erectus (Ngandong 6)
Palaeontologists' pulse rates normally go up only when a new species or "the oldest" of something is claimed. Homo erectus in Java was thought to go back to around 1.8 million years to have died out around 300,000 years ago. It was thus more than a play with figures that set pulses racing when Homo erectus remains from Ngandong and Sambungmacan, central Java in Indonesia were found to be "the youngest", dated to a mere 30,000 years ago. Homo erectus was a contemporary of fully modern Homo sapiens! (Swisher C.C. et al, 2000).
It also means that one more species had, obviously, made it through the Toba bottleneck.
The late dates were obtained by Carl Swisher of the Berkeley Geochronology Center and colleagues. (ref. Swisher et al., 1996). It added greatly to the already lively debate between those who favor an out-of-Africa model and those who adhere to a multiregional one. The former believe modern humans developed in Africa 150,000 to 100,000 years ago, then dispersed into the Middle East and Europe, where they replaced Neanderthals by 30,000 years ago, and into Asia, where they replaced Homo erectus.
The moment of discovery for Ngandong 6: the skulls' discoverer C. ter
Haar is about to remove the skull on 13th July 1932.
The eastern part of Java, Indonesia,
Fig. 5-11. The Homo erectus (Ngandong 6) skull,
was dated to 25,000-53,000 years. This was a sensation since the species to
which it belonged had been thought extinct for 300,000 years (see fig 5-4 ).
(ref. Swisher III C.C. et al, 2000).
(photographs curtesy of http://www.mnh.si.edu/anthro/humanorigins/ha/erec.html)
1 Homo ergaster/African Homo erectus
While the forward re-dating of a known Homo erectus species by more than 200,000 years to be contemporaneous with Homo sapiens was a sensation among the cognoscenti and specialists, the next major discovery was a headline shocker: in September 2003 a single (probably female) skeleton was discovered on the Indonesian island of Flores that belongs to an entirely new Homo erectus species: it was named Homo floresiensis. The species is tiny and has inevitably been nicknamed "the Hobbits".
Even more astonishingly, the Hobbits, erh, Homo floresiensis, were alive and well at least 12,000 years ago. The excavate remains of LB1 were 18,000 years old, too young for the skeleton to have fossilized. The "Hobbits" must have known and been known by fully modern Homo sapiens for tens of thousands of years. There certainly are tales of "little people" among the inhabitants of Flores and these will now have be collected with a completely new sense of urgency and scrutinized from a completely new angle. It is much too early to say whether and how how the two species interacted, whether they avoided contact, traded or warred. Nor we do have any idea when and under what circumstances these astonishing pygmy people have become extinct - if they did.
There have long been vague and unsubstantiated stories of small people in the mountainous interiors of some of he many Indonesian islands. Nothing could be found out about them except that some seem engaged in a form of "silent trade" with the coastal population. This is a trade where one party leaves items for trade at a traditional place and then withdraws, waiting for the other party to put something of equivalent value next to it. If the offer was not acceptable, one party would withdraw its offer or add to it. Agreement was reached when both parties took the other's offering away. In this way the two trading partners could exchange goods without ever directly meeting. It has not been possible to find out whether such trading is indeed going on in Indonesia.
We, being of the Andamanese Association and interested in the short-statured Asian Negrito people, of course thought that unknown groups of Negrito people could have survived in inaccessible island interiors. This is still a possibility, but perhaps reality will turn out even more outrageous, with non-human members of the genus Homo involved! Only a year ago such a thought would have been purest science fiction, but with the discovery of Homo floresiensis this is now a distinct if still somewhat unreal possibility.
But back to reality which is breathtaking enough: the tiny dwarf people lived until the very late pleistocene alongside modern humans, they had to beware of giant monitor lizards (the "Kommodo Dragon" and an even bigger now extinct relative of the same), they hunted among other prey a dwarf relative of the elephant, the Stegodon, and found at least temporary shelter under rocks and in caves. (ref. Morwood et al., 2004).
Homo sapiens appeared in Indonesia between 55,000 and 35,000 years ago. These are important dates because they also concern the early settlement of Australia. The earliest modern humans passing through the Indonesian island chains were also on their way to become the earliest Australians. (ref. Bowler J.M. et al., 2003; Turney C.S.M. et al., 2001; Roberts R.G. et al, 1990; O'Connell J.F., et al., 2004). Not a trace of a Homo sapiens presence have been found at Liang Bua which is why all tools found have been assigned to Homo floresiensis. (ref. Morwood et al., 2004).
The last trace of Homo floresiensis so far dates to 12,000 years ago when a volcanic eruption seems to have extinguished the Floresians favourit ehunting animal, the Stegodon. Even if we assume that Homo floresiensis also died out then, that would still lave between 43,000 and 23,000 years during which the two species co-existed. No evidence has yet been found that would throw any light on their possible interaction. (ref. Morwood M.J. at al. 2004 )
Fig 5-13. Homo floresiensis, the tiniest and most recently discovered member of the genus Homo. The owner of skull LB1 (for "Liang Bua 1", the first skull found in Liang Bua cave) shown above and below is thought to have been female.
Left: a modern Homo sapiens studies her ancient and much smaller distant cousin, illustrating the difference in size between the two species. Homo floresiensis had a body height of ca 100 cm and a endocranial volume of 380 cm3. A modern average female Homo sapiens in America has a endocranianl volume of around 1500 cm3 and a body height of 160-173 cm. Size reduction is a predictable and well-known evolutionary trend for isolated populations on islands which here speaks for a long residence of Homo floresiensis on Flores. (see also Fig. 5-17 below). (ref. Brown P. et al., 2004)
Right: part of Peter Schouten's painting of what a male "Hobbit" might have looked like. (curtesy University of Wollongong and the National Geographic Society).
Fig. 5-14. Left: a photograph of the skull LB1; right: the same skull in two computer tomograph views.
The remains were dated to 18,000 years by the use of Radiocarbon and Luminescence. It is thought that the skeleton was still partially fleshed when it was covered by fine sediment. (ref. Brown P. et al., 2004).
Fig. 5.15. A range of artefacts associated with Homo floresiensis were found in Liang Bua cave. (ref. Morwood M.J. et al., 2004). These artefacts occurred down to a depth of 5.8 m with an oldest date of around 74,000 years. There is also a "big game" series of artefacts that appears only in connection with Stegodon and that lasts from the oldest dated cultural horizons, between 95,000 to 74,000 years ago until the disappearance of Stegodon 12,000 years ago. The extinction of Stegodon coincided with volcanic tuffacious silts and thus may have been caused by a volcanic eruption that may also have spelt the end of Homo floresiensis.
It may be that the Liang Bua cave was not so much a permanent home base but an occasionally used camp for selective hunting of Stegodon.
On Flores (but not in Liang Bua cave so far) stone tools associated with Homo erectus dating back 840,000 years have been found. (ref. Morwood M.J., 1998)
Skull LB1 was found in September 2003 in excavation sector VII. ("Verhoeven" in the map to the left refers to the earliest excavator in 1965). Excavations on sectors I-X took place in 1978 and 1989 by R.P. Soejono. Starting in 2001 work on sectors I, III, IV and VII was extended by an Indonesian-Australian team, including R.P. Soejono. (ref. Morwood M.J. at al., 2004).
The first Homo floresiensis skeleton as found in excavation sector VII . The arms are missing but are thought to be in the unexcavated area marked with red crosses below. (ref. Morwood M.J. at al., 2004).
Fig. 5-16. Excavation work at Liang Bua cave has been going on since 1965 . Apart from tools and bones of Stegodon, bones of frog, snake, tortoise, monitor lizards, birds, rodents, bats were found. Some would have accumulated naturally on the cave floor but some were charred which suggests the presence of fire-making humans (forest fires would be unlikely to char material on a bare cave floor). (ref. Morwood M.J. at al., 2004).
The discovery of the skeleton took place when the Australian members of the team had already returned to Australia. Such can be the luck of the draw in archaeology. Thomas Sutikna of the Indonesian Centre for Archaeology was resonsible for the very difficult handling of the find. The remains were too young to have fossilized and the bones in the damp conditions are described as being like "mashed potatoes with the consistency of wet blotting paper". Less experienced excavators might not even have noticed what the "mashed potatoes" actually were. The securing and recovering the find was a difficult process but was handled so well that it has inspired much justified national pride in Indonesia. (ref. Dalton R. 2004).
The great chance implicit in the relative freshness of these bones is that unlike much older remains, they will still contain DNA that can be extracted and analysed.
Endocranial range (brain size) and Body height ratio for major groups of the genus Homo:
1. Homo sapiens
2. Erectines (Homo erectus etc.)
4. Homo floresiensis
The discoverers of Homo floresiensis think that their now famous finds were dwarfed descendants of Javanese Homo erectus and part of an endemic island fauna. The species' relative brain/body size is extreme and even outside the range of the Australopithecines - as a result of their extreme miniaturization. This, too, could be taken as an additional argument in favour of alternative 3a in Fig. 5-12 since such an extreme adaptation takes a long time.
Fig 5-18. Flores island is located in the eastern part of Indonesia. Note the famous Kommodo Dragon National Park to the west of the island. The huge monitor lizards (varans) are now limited to the National Park but when Homo floresiensis lived on the island, they were one of the chief hazards of life there. There was another, now extinct, monitor lizard species then living there that was even larger than the Kommodo Dragon.
Since Flores, as far as is known, was never other than an island, the earliest arrival of the genus Homo needed to have some form of water craft to bring them there. This begs the question of when Homo start ed to travel on water - nobody knows.
5-5. Winner takes all
Of the four species of the genus Homo that made it through the Toba bottleneck, only one is left today: Homo sapiens.
It was not the number of individuals that brought "victory" (if victory is what it is). It was an obsessive interest in technology.
What Homo sapiens is now doing is that he
(1) continues his fascination with technology,
(2) has begun to multiply to vast numbers, and
(3) tries to continue his expansion to the planets
Fig. 5-19. The number of Homo sapiens on earth since the beginning of the Holocene (the end of the ice age).