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It’s 1912, and an amateur archaeologist named Charles Dawson came forward with a discovery that almost changed our understanding of human evolution. Almost. Dawson’s find was some unusual human-like skull fragments, and he showed them to the Keeper of Geology at the British Museum, Sir Arthur Smith Woodward. He explained that the fragments were given to him by some gravel quarry workers near the town of Piltdown in England a few years earlier. So the two men returned to the site and find some additional animal fossils that provided an estimated age for the skull bones: Early Pleistocene, making it the oldest evidence of a human ancestor ever found. The excitement then continued with the discovery of a lower jaw that seemed to match the skull. But it’s teeth were odd; the molars showed patterns of wear that were similar to those found in humans. And a canine discovered later at the same site was intermediate in size between ape and human canines. With its large cranial capacity -- close to that of modern humans’ -- Dawson’s fossil closely matched what experts thought a human ancestor was supposed to look like at the
time. It was named Eoanthropus dawsoni and was hailed as a “missing link” in our family tree. And this fossil was also … fake. Totally made up. What’s known today as the Piltdown Man hoax is probably the most famous evidence you’ll ever find that, even in science, bad ideas take a long time to die. In this case, the idea was that of the Missing Link -- the idea that somewhere out there, there had to be some hypothetical specimen that would partly resemble an ape but also partly resemble a modern human. But, there is no missing link in our lineage, because that’s not how evolution works. And the fact is, the search for the Missing Link was just one phase in the evolution of our thinking about … human evolution. The idea of a “Missing Link” has fascinated people for as long as we’ve been studying our origins. But most scientists today don’t use that term to talk about the fossil record, because evolution is a lot more complicated -- and way weirder -- than just a series of species lined up single-file. So, the changing ideas about the existence of a “Missing Link” are closely tied to
how the prevailing thinking about evolutionary theory in general has changed. The oldest model for thinking about change over time was known as orthogenesis, which presents a linear model of evolution, with species progressing from one form into the next toward some kind of goal or ideal. And in the early 19th century, evolution was thought to be linear, based on the premise that species evolve from other species, and that forms between the two should have physical traits that resemble both the older and the younger forms. Linear evolution is usually depicted as the so-called March of Progress, an image that’s still very popular today. And the idea of linear evolution made it easy to accept the Piltdown Man as a ‘Missing Link,’ because it reflected that logic: It was a creature that was halfway between a human and other great apes. But the PiltdownMan hoax overshadowed the real, legitimate fossil discoveries that were being made around the same time, all over the world. For instance, there was Java Man found in Indonesia in 1891, and Peking Man
discovered in the 1920s in China, both specimens of Homo erectus. These finds were largely overlooked, because they didn’t fit another bad idea of the time --- namely, that the first modern human feature to evolve was our large brain. Compared to the so-called Piltdown Man, Java Man and Peking Man had small brains and smaller, human-like canines. And not only were these Asian discoveries dismissed, but Africa was also left unexplored in the search for human origins. The reasons for this had more to do with social politics than science, including the widespread bias against the notion that the first humans were Africans. But in 1924, while Piltdown Man was still enjoying considerable fame, limestone workers in the town of Taung, South Africa discovered a fossil that they presented to anatomist Raymond Dart. It was the tiny partial skull and endocast of a juvenile primate, which Dart called the Taung Child. He eventually gave it the scientific name Australopithecus africanus, the southern ape from Africa. And for a while, Dart himself wondered whether this child represented a missing link.
But the evidence he found flew in the face of conventional wisdom. It was clear from the fossil that the Taung Child had a small brain, and human-like teeth -- the exact opposite of Piltdown Man. The discovery was widely criticized among European researchers -- including, unsurprisingly, the supporters of the Piltdown Man hoax. But the Taung Child’s strange combination of features, along with those other finds from Asia, convinced some prominent anthropologists that maybe our evolution wasn’t as linear as they’d thought. Adding to the debate, major doubt was cast on Piltdown Man in the 1930s when a geologist mapped the gravel around the site and closely correlated it to the River Thames deposits. So how could an ancient hominin be buried in modern sediment? Finally, in the 1950s, Piltdown Man was, at long last, debunked as a fraud. Fluorine tests, and later, carbon dating revealed that the fossils weren’t from the Early Pleistocene -- they were more like 600 years old. And the jaw probably came from an orangutan whose teeth had been filed to look like it was part-way between an ape and a human.
Using microscopes, researchers revealed miniscule scratches left behind by the file on the unnaturally flattened molars. The Missing Link was never missing; it turns out, it had never existed at all. As more fossil discoveries were made throughout Africa in the mid 20th century, the linear model of evolution was abandoned. Instead, these finds gave rise to a new way of thinking: phylogeny, which depicts evolutionary relationships as branching trees. This Branching Tree model took a slightly more complicated approach to human evolution, acknowledging that not every fossil discovered was directly ancestral to us. Also that ancestor and descendent species could overlap in time. Additionally, this model demonstrated that there were many branches off the main trunk, resulting in ‘evolutionary dead ends’ -- species that went extinct without leaving any modern descendants. These side branches included sister taxa like the Paranthropines from East and South Africa. Who were robust hominins with large teeth and wide faces that did not fit the criteria
to be our ancestors. So the Branching Tree model was an improvement over the old March of Progress. And for a while, we thought it was what human evolution looked like. But then at the turn of the 21st century, new technology for sequencing ancient DNA - like from Neanderthals and Denisovans - came along. And new fossil species were discovered that lived at the same time, like Australopithecus sediba, which overlapped in time with Homo habilis. These developments revealed that our family tree was a lot more complicated than just a branching tree or bush...and that it was time to update our model. It’s now called the “Braided Stream”, a new way of thinking about human evolution that takes into account our interbreeding with other hominin species. The Braided Stream is something that biologists and geneticists have seen in animals for years, but it has only recently been applied to humans. It depicts evolution as a series of channels that sometimes branch off each other, but also sometimes reunite at various points. And as different lineages shared genes, it resulted in hybridization.
This model captures crucial information that was missing from the Branching Tree model: It shows how species can reconnect after their initial split from a common ancestor. And it highlights the importance of gene flow in our evolution as a species. So instead of searching for missing links in the fossil record, the search has begun instead for fossils that show evidence of gene flow, physical proof of what we know from our DNA, that all of us are a product of interbreeding. And even before DNA sequencing, some researchers used experimental studies to figure out what hominin hybrids might have looked like, and to determine if we may have already found some in the fossil record. Experimental models on baboons, for example, have shown that hybrids tend to be larger than either of their parents, and they tend to have large teeth and extra bone-joints, called sutures, in their skulls. So based on studies like these, researchers think we have already found fossil human hybrids! A jawbone found in Romania dating back some 40,000 years, for example, has been found to have had both Human and Neanderthal ancestry.
Experts could tell this not only because of variations in its teeth and jaw, but it was later confirmed with its DNA, which showed that the hominin had a recent Neanderthal ancestor four to six generations back. So, the Braided Stream model accounts for the fact that some hominins didn’t view other types of hominins as that different. And because of our close evolutionary relationships, we were able to reproduce with them and carry our combined genes into the next generation. The evolution of evolutionary thinking has gone from a simple linear model to an increasingly complex branching tree and now a braided stream. And the more complex we realize that our evolution has been, the more remarkable it seems! It’s the product of multiple populations, over millions of years, existing together, and exchanging genetic material. It aided our process of adaptation that allowed us to thrive in a range of environments all over the world. Through the combination of DNA studies, fossil discoveries, and experimental data on hybridization, we’re still trying to create the most complete depiction possible of our human family.
And now we can all stop looking for -- or inventing fake versions of -- a ‘Missing Link,’ because we know that a fossil that is ‘half human and half ape’ simply does not exist. It’s taken a hundred years and many changes in our thinking, but we’ve come a long way from Piltdown, the greatest hoax ever to rock the world of human origins - the missing link that wasn’t. Thanks to this month’s Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng, and Steve. If you’d like to join them and our other patrons in supporting what we do here, then go to patreon.com/eons and make your pledge! And also thank YOU for joining me today, in the Konstantin Haase Studio. If you want more adventures in deep time, just go to youtube.com/eons and subscribe.
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