Part III: Later Hominin Evolution, Part I

Chapter 14: The Origin of Modern Humans: Background and Fossil Evidence

This chapter deals with the evolutionary transition from Homo erectus to Homo sapiens and the scientific arguments to how this transition has occurred. Anthropologists agree on certain anatomical and behavior changes in this transition, namely a decrease in skeletal and dental robusticity, modifications in locomotor functional anatomy, and an increase in cranial volume for anatomy; and the increase in diverse tool technologies, more efficient foraging strategies, more complex social organizations, full development of spoken language, and artistic expression for behavior.

The two extreme viewpoints within paleoanthropology of the transition that took place is that on one side a gradual change within all populations of Homo erectus took place simultaneously, leading to the appearance of Homo sapiens in Africa and Eurasia. On the other side, modern humans have a single African origin that spread out across the old world and replaced non-modern populations. These extreme viewpoints are supported by different anatomical, genetic, and archaeological evidence with the weight of the evidence currently supporting a form of the single-origin model. Chapter 14 covers the anatomical evidence.

Two scenarios exist for the evolution of Homo in explaining how Homo sapiens came into being. In, the “Homo erectus as general ancestor” scenario, Homo as a new grade of hominin was established in Africa about 2 million years ago, and eventually spread itself across the old world. Having this large and widespread population, considerable local variation can be expected that could largely be minor, but certain major adaptations could be transmitted – either thru gene flow or cultural transmission – to homogenize the larger population. Speciation does not occur because of the large amount of minor variations taking place, but the overall homogenous effects for the whole population would serve as the evolutionary change. In this model, major behavioral differences would not be developed due to the homogenization across the larger population through genetic flow and cultural transmission.

The second scenario, “Homo as separate local trajectories” posits that H. ergaster dispersed out of Africa at a later point into relatively small, isolated populations. These populations would be prone to greater differentiation through drift or selection, but also more prone to extinction. This would cause each population to be more prone to form species boundaries and for local evolutionary trajectories to be independently established – recognizing H. erectus and H. ergaster as the result of evolutionary divergence. H. sapiens, then, would evolove from one of these trajectories. In this model, behavioral difference would become very distinct due to local isolation, leading to several modes of hominin behavior being present at all times – the long-term effects of this being a major component of later human evolution.

Competing Hypotheses for Modern Human Origins

The multiregional hypothesis originated in the 1940s from German anatomist Franz Weidenreich. He believed that anatomical diversity evolved from distinctive traits in different geographical regions from the time of H. erectus through modern humans, which he referred to as regional continuity. Recently, Alan Thorne and Milford Wolpoff have built on the multiregional hypothesis to what is now known as multiregional evolution hypothesis, which sees the transformation of H. erectus to H. sapiens as a “balance between the maintenance of distinctive regional traits in anatomy, through local regional adaptation, and the maintenance of a genetically coherent network of populations throughout the Old World, through significant gene flow” (p. 372). For this to be true, three expectations should hold true: anatomically modern humans will appear in the Old World during a similar period, transitional fossils (archaic-modern) should be found in all of the Old world, and in each Old World region, continuity should be apparent for the anatomy of ancient to modern populations.

The single-origin hypothesis dates to the 1960s from Louis Leakey, who believed that Middle Pleistocene hominins from Africa are better models than the H. erectus fossils of Asia, which he believed to be evolutionary dead-ends. Chris Stringer builds on the single-origin model of the replacement of archaic populations by invading modern populations by believing that today’s regional anatomical traits are the result of adaptation and genetic drift locally in the past 100,000 years. For this to be true, four predictions should hold: anatomically modern humans should be found in Africa much earlier than other areas, transitional fossils from archaic-modern should only be found in Africa, traits distinguishing modern geographical populations should show no links to early populations in the same region, and there should be little to no evidence showing hybridization between archaic and early anatomically modern populations.

Chronological Evidence

Specimens of anatomically modern humans from Africa and the Middle East date to be significantly older than those found in the rest of the Old World. The oldest recorded modern human population comes from the Middle Awash region of Ethiopia dating to 130,000 years ago, found by Tim White in 2003, naming the subspecies Homo sapiens idaltu. In the Middle East, Israeli cave sites have yielded specimens dating close to 100,000 years ago although retaining some archaic features. Nevertheless, scientists claim the specimens to be essentially modern. Asian sites have yielded older anatomically modern specimens, but their origin dates are widely argued and rejected, leading most scientists to believe the specimens to be only 60,000 years old. There is therefore a chronological gap that exists of tens of thousands of years between specimens found in the African/Middle Eastern region and those found in the rest of the old world, therefore giving claim to the ‘out of Africa’ hypothesis.

The Question of Regional Continuity

Because the multiregional hypothesis depends on regional continuity, scientists must find proof of such continuity in the fossil record – a contentious subject, to say the least. Figure 14.5 (p. 376) nicely lays out the chronology of anatomically modern humans region by region, which strongly supports the ‘out of Africa’ hypothesis.

- As the authors lay out each argument of each fossil find (pp. 377-391), does this support the argument that scientists are basing their observations of fossil finds

on a priori premises? Does this apply to the lumping vs. splitting debate as well?

The authors state the importance of voices outside of the current debate in interpreting which hypothesis is correct. Both Aiello and F. Clark Howell are cited as siding more with the out of Africa theory, but that no hypothesis yet can completely explain the observed fossil evidence. Further, Marta Lahr’s study of cranial features concludes that no substantial morphological evidence exists to support the multiregional model, as well as Diane Waddle’s matrix correlation tests.

The Place of Neanderthals in Human Evolution

The authors discuss the importance of Neanderthal findings because of the early discovery in the Neander valley in 1856 (three years before the publication of Origin) and the substantiation they gave to the debate of modern humans as evolved animals. Although the argument of Neanderthals being the ‘missing link’ has long been abandoned because of the subsequent findings of older hominin forms, the Neanderthals remain important in the history of paleoanthropology as a ‘test-bed for many ideas about the human evolutionary past’ (p. 395). One such important argument was put forth by Lewis Binford’s analysis that Neanderthals were behaviorally very different from modern humans, and that the contrast between the Middle and Upper Paleolithic shows the change from non-human to human behavior, or the “cultural revolution”. The current view holds that modern humans and Neanderthals diverged at some point within the last half million years and recorded parallel evolutionary histories – making the two groups two separate species, Homo sapiens and Homo neaderthalensis. The genetic evidence of chapter 15 substantiates this theory.

-Does the historical importance placed on studying Neanderthals help show the extent of western-centric bias in the sciences?

Chapter 15: The Origin of Modern Humans: Genetic Evidence

The Impact of Molecular Evolutionary Genetics

Genetic evidence has begun to show just how complex the event of the origin of modern humans really was, and in turn how simple the hypotheses based on fossil evidence really are. Douglas Wallace first studied this in the 1980s by focusing on mitochondrial DNA, inspiring the famous ‘mitochondrial eve’ hypothesis. This posits that all mitochondrial DNA of living humans can be traced back to a single female living in Africa approximately 200,000 years ago. This female would have been in a population of about 10,000 individuals, all relating to the foundation of the modern human population who spread across the old world and replaced all existing archaic species of Homo. This data was deemed inadequate during the 1990s (see p. 405 for a detailed analysis), with genetic work shifting to two pathways: one extending mitochondrial DNA analysis to include other genes (including ones from the nucleus) to determine when and where humans evolved, and the other using mitochondrial DNA to infer population dynamics of early modern humans – both supporting the single-origin ‘out of Africa’ claim.

Mitochondrial DNA is useful for understanding recent evolutionary events for two reasons: first, because it accumulates mutations ten times faster than in nuclear DNA therefore providing more information over the short term; and second, because it is inherited only from the mother, it substantially cuts through the fog that recombination (maternal and paternal) genes create. The evidence collected from mitochondrial DNA supports a recent bottlenecked, single-origin model of the human lineage.

The problem with studying mitochondrial DNA is that it provides a gene history, not a population history. The mitochondrial is just one special gene and scientists now realize the need to study other genes to search for evidence to substantiate evolutionary claims.

Recent Developments

Scientists are now looking at two new types of genetic data, the first being microsatellites which are rapidly evolving short stretches of DNA of two-to-five-nucleotide segments. David Goldstein has used this technique to find a coalescence point of origin of approximately 156,000 years ago. The second is Alu elements, which are sequences of DNA approximately 300 base pairs long that are inserted in large numbers over the nuclear genome. Once these are inserted, they are never removed and therefore remain immune to homoplastic changes. A recent multi-authored analysis has resulted in a coalescence time of 102,000 years.

Another study focuses on the fact that because mitochondrial DNA helps give the history of women, the Y sex chromosome will help give the history of men, because only men possess it. Stanford University scientists are using the Y chromosome to obtain a coalescence of between 100,000 and 150,000 years, with a pattern showing greater African diversity than the rest of the world’s modern populations.

The importance of coalescence times in different genetic studies shows how some genetic systems have older patterns of diversity, even if there was a ‘harsh population bottleneck’ of 150,000 years ago that is supported by this evidence. Therefore, studying the complexity of different genes and their coalescence times will help broaden our understanding of our overall population history.

This has led to the study of the bottlenecking of modern human populations over history, with the severe reduction coming from different environmental events followed by the explosion in numbers of individuals in the population. Certain hypotheses have been put forth supported by genetic evidence, including the Garden of Eden hypothesis (figure 15.14, p. 415), which posits that humans fragmented within Africa forming distinct genetic populations. Subsequent population bottlenecks within Africa led to ‘proto-‘ European, African, and Asian populations that then began to spread across the old world. The authors deem this argument to be very weak, but nevertheless one of the many hypotheses developed in the out of Africa model. Figure 15.16 (p. 419) further illustrates the evidence of the history of bottlenecking in human evolution and the relation between genetics and population dynamics – all supporting the out of Africa hypothesis.

-As the field of molecular genetics expands in trying to explain the origin of modern humans, what can be learned from the past in forming sound hypotheses?

Chapter 16: The Origin of Modern Humans: Archeology, Behavior, and Evolutionary Process

This chapter explores the archaeological evidence that would help support the claim that modern human behavior would start first in Africa and spread out to the rest of the old world. This archaeological evidence should substantiate the link between behavior and biology in the history of modern human populations and whether biology led to behavior or vice versa. However, a gap exists in these findings because the small amount of sites in Africa (only about a dozen) in comparison with other sites (SW France, for example with 100 sites, but 100 times smaller than the area in East Africa). This can lead to misinterpretations (Eurocentric, specifically) because of the lack of evidence in the archaeological record, so much work needs to be done to further the out of Africa hypothesis from the archaeological and behavioral standpoint.

The dates in question are the Middle Stone Age and Late Stone Age in Africa -- 300,000 to 30,000 years and 30,000 and 10,000 years, respectively. The equivalent stages Europe, Asia, and North Africa are known as the Middle Palolithic and Upper Palolithic, and all deal with the tool assemblages of specifically anatomically modern human populations first explored in Chapter 12. Figure 16.7 (p. 430) nicely summarizes the findings of these stages in different areas of the old world and the tie between the origin of Homo sapiens, the environment, and complex behaviors.

Regional Patterns in the Archeology

European evidence: The transition in Europe is known as the Upper Paleolithic revolution, which coincides with the first appearance of modern humans to the region that carry the cultural traditions that are known as Aurignacian. This happened about 40,000 years ago, according to radiocarbon evidence in northern Spain and it includes blade-based technology and the use of bone, ivory, and antler for points as well as body ornamentation. As the upper Paleolithic progressed, so did temporal and spatial variability of style and the foundations of cultural traditions as we know it today for the Aurignacian.

Asian evidence: The Middle East provides evidence as a natural migration route between Africa and the east and evidence exists from at least 200,000 to 50,000 years ago of Neanderthal, early modern, and other hominin populations. The scarcity of sites in Asia make it difficult for scientists to interpret behavior and tool use, but evidence does suggest significant differences between the east and the west of the continent – further evidence for the out of Africa hypothesis, with the possibility of multiple dispersals of hominin populations.

African evidence: Sub-Saharan Africa should hold the evidence for the evolution of modern human behavior for the out of Africa hypothesis to hold true. This is important not only for finding the earliest date of modern human tool-using behavior, but also include transitional steps in line with dispersals to other parts of the world. In sum, archeological evidence should mirror scientific presuppositions. Figure 16.11 (p. 439) shows the evidence collected towards explaining ‘modern’ African behavior – but the archeological record is very sparse in comparison to Europe, leading scientists to find very little early symbolic behavior in contrast to European findings. This is because of, as Alison Brooks (and the authors) argues, the unfavorable conditions for preservation in Africa and the little amount of sites actually investigated.

-Although the anatomical and genetic evidence is strong for the out of Africa hypothesis, does the lack of archaeological evidence give credence to the multiregional hypothesis believers? Is it fair to assume past African behavior although the archaeological evidence is by no means complete?

Toward an Integrated Model of Modern Human Origins

The authors argue for a ‘multiple-event, multiple-dispersal’ model to build on the out of Africa hypothesis. This is based on a few interrelated premises supported by evidence presented thus far:

- All living humans are descended from a relatively small population from Africa (in all probability) dated to approximately 150,000 years ago.

- Hominins outside that population, be they archaic or modern, have not contributed significantly in genotype or phenotype to the modern population – all consistent with the notion that the modern population from Africa colonized and replaced all other populations.

- The ‘origins’ of modern humans are not based on a single event, but rather spread over the last 200,000 years towards the development of anatomical modernity in Africa.

- Multiple dispersals explain the spread of modern human populations around the world.

- Modern humans share their evolutionary origins in archeological terms of the ‘Mode 3 industries of Africa’ with the Neanderthals. Behavior is shifted in cultural terms rather than biological or genetic.

- Not all archaic hominins are the same.

These premises are all related to the ‘principles of evolution’ outlined by the authors (p. 444): the importance of geography, the role of the climate, the role of behavior, the importance of microevolutionary processes, and the importance of extinction in explaining all evolutionary process and change.

-The authors discuss the need for complexity in understanding human evolution as opposed to the simplicity that Darwinism offers. Is human evolution that much more complex or are we as susceptible to the same evolutionary processes that affect all of life? Is it possible that our belief in the complexity of our evolutionary history is a reflection of our unique ecological niche as a big brained, socially dependant species? For example, what would Canis familiaris say about its own evolutionary history if it could?