Evolution of Evolution — Text-only | Flash Special Report
Human Evolution's Winding Path
Darwin got it mostly right
By Tim White
The 1859 book “On the Origin of Species” is notorious for Darwin's evasion of the subject of human evolution. In it, he wrote only: "much light will be thrown on the origin of man and his history." However, Darwin more directly discussed the subject of human evolution in his 1871 treatise, “The Descent of Man.” In that book, he clearly laid out a prediction that human ancestors would be found in Africa. Of course, at the time, there was virtually no known fossil record, so Darwin could only predict that someday, some "future geologist" would find fossils connecting modern apes with modern humans via an ancient undiscovered common ancestor. These predictions proved to be remarkably accurate.
Based on what was known at the time about living apes and humans, Darwin 'triangulated' that our earliest hominid ancestors were the result of feedback between bipedality, freedom of the hands, reduction of the canine tooth and face, expansion of the brain, and the increasing use of tools. In other words, he postulated simple, straight-line, human evolution. In the 138 years that followed, however, evidence showed this model was too simplistic as the fossil record revealed a more complicated path to modern humans.
Prehistoric stone tools were found across Europe long before Darwin and his contemporary British naturalist Alfred Russel Wallace explained evolution via the mechanism of natural selection. Accompanying some of these tools were human-like fossils that took their name from Neanderthal, the German site where they were first discovered. The “Neanderthal Men” associated with the site became one of the earliest challenges to Darwin’s view of human evolution. Neanderthals were interpreted by some as pathological, whereas others saw them as an evolutionary side branch, and yet others as direct human ancestors. The Neanderthal debate persisted for nearly 150 years, and was only recently resolved by a powerful combination of well-dated African fossils and comparisons of ancient Neanderthal DNA with DNA of modern humans. Evidence now indicates these near-humans were a circum-Mediterranean side branch that went extinct about 30,000 years ago and did not give rise to modern humans.
Neanderthals show that the path to modern humans was not the simple, straight-line predicted by Darwin, since they existed too recently to bear on Darwin’s predictions about what our much earlier progenitors were like, or where they lived. However the recovery of well-preserved fossils in a well-calibrated, dated geological sequence largely have proved Darwin’s original hypothesis about an African birthplace. These early human-like fossils first were discovered in the 1920s, in Africa, as Darwin predicted. At first there was considerable controversy over these remains. Today, however, we recognize several species that all belong within Australopithecus, a zoological genus closely related to the human genus Homo—and seemingly ancestral to it. Perhaps the most important of the thousands of Australopithecus fossils known today is a small female found in the Ethiopian desert. Her scientific name is Australopithecus afarensis, and she died 3.2 million years ago. Her bones eventually eroded out of ancient channel deposits and her discoverers nicknamed her "Lucy." The completeness and quality of her skeleton has provided considerable insight into early hominids. On the family tree, Lucy's species is one of many now bridging the gap between modern people and the last common ancestor that we shared with the living African apes.
Science has both creative and critical dimensions. Darwin, as a historical scientist, employed what he knew about the modern world to predict what might have been found in the world of the past. He creatively and correctly hypothesized that humans evolved in Africa. He creatively but incorrectly hypothesized that our earliest ancestors would appear to be a halfway house between chimpanzee and human. The evidence of Lucy and the many other fossils of the species that she represents shows that these creatures were neither chimps nor humans, nor were they halfway between modern chimpanzee and human, but rather had their own unique characteristics. This uniqueness is confirmed by evidence of the archaeological record as well as geochronology. The molecular evidence also confirms this in the living endpoints of two very different evolutionary trajectories.
Although Darwin’s hypothesis on the location of the evolution of man has proven correct, Darwin's hypothesis about the sequence in which human characteristics arose during our evolution has been rejected. It seems likely that Darwin would have been delighted to witness the arrival and interpretation of that new evidence. The approach that he brought us—how to be curious, critical and creative scientists—has indeed seen evolutionary biology through the last 150 years very nicely. Darwin's core insight lets us understand how evolution works and is now the bedrock of biology. It is also the very basis for understanding how we got here.
Tim White is a world renowned paleoanthropologist and professor of Integrative Biology at the University of California at Berkeley. His work frequently takes him to study sites in Afar, Ethiopia, Jordan, Kenya, Malawi, Tanzania (at Olduvai Gorge and Laetoli) and Turkey. His primary research involves human evolution in all its dimensions and he and his colleagues are credited with the discovery in Ethiopia in 1995 of perhaps the oldest known human ancestor, Ardipithecus ramidus, dated to 4.4 million years ago. The National Science Foundation supports his work on a Middle Awash research project in Ethiopia.
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