Thursday, June 21, 2012

What We're Reading: New Science Books


We are fortunate to be living in an exciting time for paleoanthropology. Over the course of the last 50 years or so, with new discoveries in Africa and elsewhere, an understanding of the history of human origins has been slowly emerging. There have been many revisions in the consensus concerning that history over the years, and those interested in this subject and who have not revisited it for awhile may in fact be a little startled to discover how the picture has changed.

Ian Tattersall of the American Museum of Natural History in New York has written extensively on human origins over the years. The books he has written are usually overviews of where we are today on the subject, analyzing the significance of recent discoveries, integrating them into previous knowledge, and presenting the reader with a current summary of the field. Previous books have tended to focus on the fossil record and what it tells us about differences in human anatomy and biomechanics, but in this most recent overview Tattersall has incorporated into his summation the latest findings from the disciplines of molecular anthropology and cultural anthropology as well as speculations derived from studies of primate cognition. Tattersall’s books have been noted for their evenhandedness. He has positioned himself to be a moderator in evaluating the competing and controversial claims of partisans, but the presentation here is notable in that only particular discoveries and not personalities are discussed. He focuses here on areas of consensus rather than the details of particular disagreements in order to give us a broad overview of the field at this time.

The picture of human origins in the mid and late 20th Century relied heavily on the feature of bipedality as being decisive in classifying fossils as hominids. Various hominid species were also associated with particular stone “toolkits.” Significant developments in anatomy were thought to be associated with major changes in the African landscape due to climate change, which was associated in turn with changes in diet, particularly the inclusion of meat in hominid diets. The prevailing view of how evolution worked at the time was that significant changes resulted from small incremental changes that accumulated over a very long period of time, changes that were preserved in a species because they were adaptive. The idea of incremental change fostered the notion that a rather linear line of hominid development could be traced through several successive species of hominid to Homo sapiens. Some hominid species, particularly Homo erectus were recognized as a species that had left Africa and peopled areas of Europe and Asia. While there was some thought that Homo erectus may have developed into Homo sapiens in these areas, the general view seemed to hold that the origins of Homo sapiens was in Africa, and that their eventual domination of the world was the result of a later exodus of Homo sapiens from Africa accompanied by the eventual extinction of Homo erectus. The place of Neanderthals in human evolutionary history was not completely settled. While it seemed that most paleoanthropologists did not believe Neanderthals to be in a direct developmental line with Homo sapiens, some entertained the possibility that the species had interbred with Homo sapiens and was eventually subsumed into the Homo sapiens species.

In Masters of the Planet the long held sceintific dispositon to trace some direct line in the fossil record or to find missing links has largely been discounted. For one thing, we now know that the world was populated by a multitude of hominid species living over a long period of time and in some cases overlapping with other species in time (although usually living in geographical isolation from each other). But more importantly, Tattersall, using ideas that come from new understandings of how evolution works and how species “change,” appears to suggest that there may never be something found in the fossil record that seems definitely “on the way” to being Homo sapiens. He finds Homo sapiens to be so morphologically distinct from other hominid species that he believes they were the result of some radical genetic modification, what he calls a “hugely ramifying genetic accident,” a relatively sudden event which resulted in not an immediate adaptation but rather an “exaptation.” This was not a change which resulted from a slow accumulation of changes in a particular coding gene but rather from a major shift in how other genes controlled the expression of that gene in the development of the organism. These “exaptations” represented potential adaptive pathways that were only later fully exploited. The major change in the hominid line, the change he finds unique to Homos sapiens is not the size of the brain but the way the brain is wired in this species and how that allowed for what he views as the most important difference between humans and other animals, the development of symbolic cognition. He does not believe this change occurred in other species of hominids, including Neanderthals.  It was a change that enabled the all important and unique invention of language in Homo sapiens.

In Tattersall’s view, the old narrative of hominid development---the one in which first we evolved to walk upright, then use tools, then became hunters and meat eaters to develop and sustain bigger brains and finally developed language and agriculture---is one that didn’t adequately explain hominid development. It is now a narrative that is at odds with what we know about the timetable of hominid evolution. It appears that Australopithecines probably already were bipedal in the forests and that their upper bodies were built for forest climbing. They did not have to develop bipedality as an adaptive way of exploiting a woodland environment. Meat was not, it appears, a significant part of their diet. Stone tool technology remained unchanged for millions of years and changes in the sophistication of that technology do not correlate strictly with the appearance of new and later hominid species before the advent of Homo sapiens. While there seems to be a gradual trajectory of growth in brain sizes of hominids over time, Tattersall does not see any precursor to our type of brain or human consciousness and cognition in the cultural evidence left by other hominid species, and he does not believe that language as we know it would have been possible in other hominid species. The idea that Neanderthals have any significant relationship to Homo sapiens seems to have become even more of a minority view

The implication of Tattersall’s summary would seem to be that, unlike in the early days of paleoanthropology, the kind of more definitive physical evidence about human origins we thought might be found, particularly in the fossil record, may not be the way that in the future we develop a greater understanding of human origins. Certainly some new and extraordinary fossil find could shake the tree and change the paradigm, but that find seems less likely than it did some years ago. If fossil endocasts of certain brain structures cannot be equated with how the brain functioned at a given date, as Tattersall cautions, his view suggests that our growing understanding of human origins may have less to do with new fossil evidence and more to do with how we understand how species change, with how the expression of genes are modulated by other genes in orchestrating morphology, and by how our growing understanding of the brain may suggest a pattern and history of our cognitive development. The picture has changed, but the quest to understand the origins of man remains an exciting and provocative story yet unfolding in our time.

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