Human History Twist

On the island of Flores, about a decade ago, in the Liang Bua cave there were discovered some bones. The bones were human like, but small, perhaps those of a child. Mercifully, this was not the scene of a grisly infanticide but the last resting place of an ancient human, but a human unlike any other that had been seen before it. This pile of bones would go on to be called Homo floresiensis and would controversially divide the paleoanthropology community.

The problem with H. floresiensis was that its brain was too small and its stature too short. Dating evidence showed that the individuals in question had been alive a mere 50,000 years or so ago, very recently in evolutionary terms, and all the human species of the time were anatomically the same as us. H. floresiensis, by contrast, stood at only 1 metre tall; far too short for a modern human. The question, then, was this: were the bones in the Liang Bua cave a new species of human or did they belong to a normal modern human that had a disease of some kind that rendered them short and with a small brain?

Another problem was how a small-brained and physically short human managed to survive for so long? The much larger and stronger neanderthals had been all but wiped out by then. And what was the reason for H. floresiensis’ small proportions? If it was around so recently then it probably evolved from Homo erectus, the first hominin to make the journey out of Africa. As the name suggets, however, H. erectus is a strapping specimen that stands much taller than those from Flores. Did millennia of isolation and limited resources result in a case of what is known as Island Dwarfism?

Over the decade the debates have continued, though it is my feeling that the new species camp is winning. Perhaps we’ll never know for sure until we find another site in the area with further examples? In any case, there was a new revelation about our diminutive cousin just last week.

In an open access paper published in the Journal of Human Evolution, a new analysis looked at 133 skeletal and dental characteristics from across known human species to try to firmly place H. floresiensis within the human family tree. The results were rather surprising.

The analysis concluded that H. floresiensis was not most closely related to either H. erectus nor us, Homo sapiens. The closest human species they are related to turns out to be Homo habilis. This makes sense as that pair are both quite short and have smaller brains; but this would mean that H. floresiensis is a much more ancient species then most had suspected. The logical conclusion of this is that H. floresiensis did not evolve from H. erectus during the last wave of migration out of Africa some 150,000 years ago, but that it left Africa long before then in a previously unknown human migration that happened some 2 million(!!) years ago. If this turns out to be true then it would really shake up the paleo field. The good thing is is that this is a testable hypothesis, it makes predictions that we can look out for in future research.

In other paleoanthropological news, there was another bombshell dropped this week. Homo naledi is a species of human discovered in a cave in South Africa in 2013. It was first properly described in the literature in 2015 but, unusually, the authors chose not to publish an age for the bones at that time. Though this goes somewhat against convention, they took this unusual step as they knew that it was going to be particularly difficult to date H. naledi.

There are a few main ways that you can date ancient bones. You can conduct an isotopic decay analysis of elements within the bone; researchers had initially ruled this out as it involves the destruction of very precious bone sample. You can look at the stratigraphy of the layers of rock in which the bones are found; this was not possible with H. naledi as much of the rock strata had been washed out over the years. Finally, you can look at fossils of other organisms found alongside your bones of interest. If you can date the contemporaneous bones then you can infer the age of your bones of interest that way; however, it is not thought that any other bones were found at the site.

The morphology of the skeleton provided some clues, though. Short stature, small brain case, curved fingers for gripping branches; these all suggested an ancient origin of between 2-3 million years ago. But in an interview with National Geographic magazine last week, the charismatic research leader Lee Berger revealed that the age of the bones has been pinned down to just 200,000-300,000 years ago. This would come as a huge surprise if it turns out to be accurate. Excitement must remain tempered at this early stage, though, as this work has yet to be published.

The implications of this discovery are that we must once again redraw the family tree of our ancestors. Similarly to on Flores island, a small, relatively primitive human species lived for millions of years alongside taller, stronger, smarter people. In contrast to Flores, H. naledi would not have had the sea for protection.

The dating of these bones could have another grim detail hidden within them. It was at about this time that modern humans, H. sapiens, were making their way through Africa. Could it be that H. naledi was the first in an ever increasing list of species to be wiped out by us? That is a question that might be impossible to answer with the fragmentary evidence to have survived the ravages of time since then. It certainly makes me very eager to see if we can get a useful genome sequence out of the H. naledi bones; perhaps we bred with them the same way we did with neanderthals and, even if it might have been us that led to their demise, their legacy might live on within us.

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