Repost: The species that domesticated itself

Zinj

The skull of Paranthropus boisei (“Zinj,” “Dear Boy,” “Nutcracker Man,” etc.).

Louis Leakey had a problem.

During the summer of 1959 he and his wife Mary recovered the skull fragments of an early human scattered about the fossil deposits of Olduvai Gorge in Tanzania. The skull had been deposited among the shattered bones of fossil mammals and a collection stone tools, and this led Louis to conclude that it was one of our early ancestors. Only an ancestor of Homo sapiens could be a toolmaker, Louis thought, but the skull looked nothing like that of our species.

When Mary fit all the pieces of the fossil puzzle together she and Louis were greeted with a visage that was both familiar and alien. The a low-domed skull capped a face expanded sideways by massive cheek bones through which powerful muscles attached to the jaw. Overall the new skull, affectionately dubbed the “Dear Boy”, was most similar to the skull of the australopithecines from South Africa, specifically a heavy-jawed type called Paranthropus that had been discovered decades before.

Louis recognized this similarity, but while he pushed Paranthropus to the sidelines of human evolution he presented the new fossil, named Zinjanthropus, as one of our ancestors. His mind was made up. The only serious barrier to this connection seemed to be a matter of time.

At the time “Zinj” was discovered the geologic timescale was still undergoing major revisions. Absolute dating techniques were still relatively new and many of the dates ascribed to human fossil sites were vastly different from what repeated testing has revealed them to be. In this case Zinj was thought to have lived within the last 600,000 years during the “Ice Age.” This was not so very long ago, and it stretched credulity to think that a creature as superficially gorilla-like as Zinj had been transformed into our species during so short a time.

To settle this discrepancy Leakey appealed to the stone tools that had led him to his conclusions about Zinj. In a lecture presented to the South African Archaeological Society in 1960 Louis presented what he glibly called “Leakey’s Theory”, or the idea that the invention of stone tools allowed humans to domesticate themselves and accelerate evolution. Louis said;

If we are willing to concede the possibility, as I most certainly am, that the genus Homo was derived from an australopithecine resembling Zinjanthropus, then we have to ask ourselves whether or not a sufficiently long time interval elapsed between the Lower Pleistocene and the closing stages of the Middle Pleistocene to allow for the transformation, by evolution, from Zinjanthropus to something approaching Homo.

Ordinarily the answer would be “No”, but Zinj was a special case. And, like other naturalists before him, Leakey appealed to domestication to provide a model by which evolution could proceed;

It is, I think, nowadays widely accepted by biologists and others, that man, by the act of domesticating animals and plants, speeds up the results of natural evolution, simply because he creates conditions more favourable to the survival and subsequent reproduction of aberrant mutants. Put in other words, the moment man creates, for any species, the new, unnatural conditions of life which we call ‘domestication’, he greatly accelerates the speed of the natural evolutionary processes. This is true even before he starts carrying out selective breeding.

It seems to me that a time interval of some 400,000 years is more than ample for the evolution of a creature like Zinjanthropus, once he domesticated himself, into Homo … In fact, once he had become the maker of stone tools, there is no reason at all why human evolution should not have been as rapid as that of his many subsequent domestic animals.

If this sounds a little bit fuzzy, it certainly was. For Louis domestication and artificial selection did not simply provide an illustration of how evolution might work but were mechanisms that sped up evolutionary change; how else could we wind up with so many dog breeds so fast? It was not a well-reasoned argument, but it did not necessarily have to be. Everyone knew that only humans made and used stone tools, and therefore Zinj was a human despite failing to meet every other expectation previously held for a direct human ancestor.

At about the same time as Louis was delivering his 1960 address, however, his son Jonathan was finding bones that would undermine the vaunted position promised to Zinj. There was another fossil human at Olduvai, one that was more anatomically similar to us in form than Zinj, and so Louis quickly changed his mind. Within a year Zinj was booted to an evolutionary “side branch” while the new fossils took pride of place in Louis’ mind as our ancestors.

But Louis could not publish his revised conclusions right away. His impetuous nature was well-known and to change positions so quickly could be hazardous. He had already academically embarrassed himself twice by claiming to find human ancestors that were nothing more than recent burials (to say nothing of the unpleasantness of his leaving his first wife with child to elope with Mary), so he waited for more extensive fossil evidence to back up his hypothesis.

Even so, Louis was itching to describe the new fossils, and by 1963 he was growing impatient. At least one of the co-authors of the paper describing the new human, Philip Tobias, thought the new fossils might represent an australopithecine and not a member of our own genus, but by 1964 Tobias relented. Together Leakey, Tobias, and John Napier announced the oldest recognized member of our own genus, Homo habilis, the crafty human who had created the stone tools and may have even murdered poor old Zinj. (Zinj would later be placed as a species of robust australopithecine, Paranthropus boisei, which lived between ~2.6 to 1.2 million years ago.)

While certainly able in the field, Louis Leakey’s ambition to find the first true human ancestors often ran away with him. It seems that in Louis’ view he and his team discovered our ancestors while everyone else was puttering about with evolutionary dead-ends. Once he had in mind that something was an ancestor, such as Zinj, he forcefully made the case that it was so, even if he had to abandon the very notion he had just popularized. Nor are such maneuvers entirely things of the past; paleoanthropology is still fraught with strong personalities that are constantly bickering over who has discovered our true ancestors.

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