's sober Fossil Find Points to a Streamlined Human Lineage
, to The Guardian
's Skull of Homo erectus throws story of human origins into disarrary
, the news media were pretty much in agreement: a new article
about human ancestors from Dmanisi was revolutionary.
Scholars like John Hawkes agree these are important fossils, but they are much less likely to pronounce the end of all previous understandings of human evolution.
Many of the news stories described the Dmanisi finds as a population, with The Guardian specifying that the five individuals represented were "most likely an elderly male, two other adult males, a young female and a juvenile of unknown sex." That certainly sounds like a population: male and female, adults of multiple ages and even a young person.
But using the word "population" may be a little misleading for readers of the popular press: these five individuals died in the same place over a period of what is estimated to be hundreds of years, around 1.8 million years ago. So, not "population" in the sense of people living in the same place at the same time. Population in the sense of a series that scientists can validly study as a group—in this case, on the assumption that they were members of a single species living in a specific location for some generations.
Dmanisi is in Georgia, and one of the media narratives pits the skulls found there against the fossil record from Africa. NPR wrote
The conventional wisdom about early human evolution has it that there were several species that arose in Africa: Homo rudolfensis, Homo habilis, Homo erectus and maybe even more. Now this new discovery suggests that a single species, exemplified by the Dmanisi Five, can have more physical variety than previously thought. In fact, the team found as much variation among modern humans and among chimps and among the Dmanisi Five as there is among those ancient African fossils that have long been thought to be different species...
"There may have been one very successful species that emerges from Africa... and rapidly spreads to Southeast Asia. That's a picture of a very successful, cosmopolitan species." It's different from the notion, held by many, of a wide range of early-human-like forms that emerged independently, and eventually were culled until only one was left standing.
The Guardian quotes an alternative argument:
I think they will be proved right that some of those early African fossils can reasonably join a variable Homo erectus species...But Africa is a huge continent with a deep record of the earliest stages of human evolution, and there certainly seems to have been species-level diversity there prior to two million years ago. So I still doubt that all of the 'early Homo' fossils can reasonably be lumped into an evolving Homo erectus lineage. We need similarly complete African fossils from two to 2.5m years ago to test that idea properly.
The scholarly point: the group of skulls from what can be assumed to be a series of related individuals is highly diverse. The amount of diversity within this group is comparable to the amount of diversity within the contemporary hominid fossils from Africa—but because those were not found in one place, each new find has been an opportunity to name a new species.
But what does this tell us about the topic the news media are interested in: the story of human origins, the streamlined lineage?
Hawkes focuses on what the Dmanisi fossils tell us. The underside of the cranium draws his particular attention: preservation of this part of the skull, he says, is rare, and this one—D4500—is the most intact. (A good series of images can be found here.) He discusses, carefully and yet accessibly, the comparisons between the Dmanisi skulls and examples from Africa.
And then he adds his own thoughts on the bigger picture—the controversy, as the media see it.
The essential claim: the shape variation among these four Dmanisi crania is approximately the same as the shape variation among all East African early Homo. Moreover, measured on the same axes, the shape variation among all these early Homo specimens is approximately equivalent to the shape variation among living humans, or the shape variation within living chimpanzees....Not only does early Homo look like a single lineage when we consider how it changes across time—the entire early Homo sample has barely more variability than is found within the single Dmanisi site, at a single time. But both comparisons arrive at the same conclusion: early Homo does not look like a group of radiating species, it looks like a cohesive lineage.
This then leads to a wonderful discussion, which I cannot do justice to here (go read it!) in which Hawkes takes the variability seen both in and out of Africa and shows that we need to understand it as evidence of a complicated history:
What population structure characterized the African ancestors of the Dmanisi hominins? ... the structure would be one of multiple populations, strongly differentiated, that had existed for hundreds of thousands of years. ...they shared genes and one might occasionally replace another, only to re-differentiate as climate fluctuated. ... Some of the remixture between them would have been slight, on the scale of Neandertal mixture into today's human populations. But those cases were at one end of a continuum that included larger amounts of genetic exchange...
This pattern of similarities cannot be easily explained under a model of bifurcating change. It implies some degree of continued dispersal and mixture. I expect that the dispersal and mixture were biased in direction, probably more moving out of Africa than back into Africa. The dispersal and mixture happened episodically, not continuously.
This is a very different way to understand the implications of Dmanisi: human ancestors living, not as local, highly isolated populations, but as a river moving primarily out of Africa, but also back; ancestors we have to think about not simply from each locality, but on a global scale.
The simpler story may be easier to understand. But it is far less likely to be a good explanation.