Man bites dog08 Aug 2007
Appropriate to yesterday's post about the hypothesis of a Eurasian-African clade distinction in early humans, is today's paper from Fred Spoor, Meave Leakey and others, describing the KNM-ER 42700 calvaria and the (unassociated) KNM-ER 42703 maxilla.
The cover photo from the issue is brilliant -- a juxtaposition of KNM-ER 42700 and OH 9 at the same scale:
I wrote about KNM-ER 42700 a couple of years ago, when it was shown at the meetings. A few things have changed since then. Most important, the specimen is now accepted as an adult, so that it is assumed to have reached its full adult brain size. That also means that the supraorbital torus, angular torus, and other features reflecting robusticity were probably near their maximum development.
I have much to say about this and the other fossil, which the paper attributes to Homo habilis. The press accounts have all led with the (very) uninteresting and conventional. Here's the AP's Seth Borenstein:
The new research by famed paleontologist Meave Leakey in Kenya shows our family tree is more like a wayward bush with stubby branches, calling into question the evolution of our ancestors.
The old theory was that the first and oldest species in our family tree, Homo habilis, evolved into Homo erectus, which then became us, Homo sapiens. But those two earlier species lived side-by-side about 1.5 million years ago in parts of Kenya for at least half a million years, Leakey and colleagues report in a paper published in Thursday's issue of the journal Nature.
Here's John Noble Wilford:
Two fossils found in Kenya have shaken the human family tree, possibly rearranging major branches thought to be in a straight ancestral line to Homo sapiens.
Scientists who dated and analyzed the specimens - a 1.44 million-year-old Homo habilis and a 1.55 million-year-old Homo erectus - said their findings challenged the conventional view that these species evolved one after the other. Instead, they apparently lived side by side in eastern Africa for almost half a million years.
Here's Robert Mitchum in the Chicago Tribune:
Two small fossils unearthed in Kenya - the top of a skull, and half of a jawbone - fill an important gap in the evolutionary story of how humans came to be, yet have created as many questions as they have answered.
The similar age and location of the fossils suggest that two early humanlike species, Homo habilis and Homo erectus, closely coexisted rather than coming one after the other on the evolutionary road to modern man, according to a paper published Thursday in the journal Nature.
I could go on. They write themselves, don't they?
But this idea of contemporaneity of H. habilis and H. erectus is neither interesting nor new. Recall yesterday's story about the African and Asian clade hypothesis? News stories had the same lede -- "hominid family tree more complex than thought." This is the ultimate paleontological "dog bites man": "Human Evolution A Bush, Not A Ladder." It's just not interesting anymore.
Why is it old news? Well, we could look back at Bernard Wood's 1991 Koobi Fora monograph, which went into long detail about the assignment of fossils to Homo aff. H. erectus -- fossils that in every case were older than the latest occurrence of Homo habilis at Olduvai.
At least, they thought they were older...
You see, there's some really interesting stories to be told about these fossils. Stories that hasn't appeared anywhere in the press.
Here's a question: Why does that small KNM-ER 42700 skull have all those cranial features from much later, larger, Asian Homo erectus skulls?
Here's what Spoor et al. wrote about it:
The presence of supposedly distinctive 'Asian' characters , such as cranial vault keeling and a well separated petrous crest and mastoid process in KNM-ER 42700, underscores the difficulty in separating the African and Asian hypodigms of H. erectus . This difficulty is further accentuated by the observation that the more angulated occipitals and the thicker vaults and supraorbital tori seen in Asian H. erectus are allometric consequences of an increase in cranial size, rather than independent characters (Spoor et al. 2007:689).
Of course, the answer is that they aren't really Asian features. That much is evident from the fact that the later African skulls, OH 9, BOU-VP-2/66 (Daka), and Buia, also have many of them.
KNM-ER 42700 demonstrates that the traits were present in African H. erectus almost from its earliest occurrences. If these early Africans shared the same features as early Asian Homo erectus, then the hypothesis (promoted by many) that these early Africans are themselves an entirely different species, called Homo ergaster must be wrong.
At last, sinking one of those new-fangled bushy human species, and for good? Now, that sounds more like "man bites dog!"
But wait, there's more! Last year, Frank Brown's geochronology group redated many of the early Homo specimens from Koobi Fora, with the surprising result that early Homo erectus no longer included any cranial fossils that were demonstrably older than 1.65 million years. Here's what I wrote at the time:
Looking at what is left in the early part of the sequence is certainly interesting, but just as interesting is how all the H. erectus-like specimens are all bunched together between 1.65 and 1.45 Ma. This is the time interval that already held KNM-WT 15000, KNM-ER 3883, and KNM-ER 42700, and is just older than OH 9. Now we can add KNM-ER 3733, KNM-ER 730, KNM-ER 1808, and KNM-ER 1821. Isn't this an interesting sample? Don't you wish we knew about the other postcrania?
It seems to me that the hypothesis that H. erectus-like hominids first appeared in Africa around 1.65 Ma has interesting archaeological consequences. This isn't long before the appearance of the earliest Acheulean, and it plausibly makes the Developed Oldowan-Acheulean sequence a correlate of this evolution.
It is markedly not coincident with the earliest such evidence in Asia. But that raises the Dmanisi question again, doesn't it?
This is an amazing problem, now. The consensus that Homo habilis and Homo erectus overlapped in time was thrown completely open by the redating. This paper by Spoor and colleagues, by presenting both a new H. erectus specimen and a very late H. habilis specimen, was directed toward this problem. If they are right, it re-establishes the status quo: Homo habilis hung on after the evolution of early Homo erectus, the two species being radically different in their body size (and presumably life history) adaptation, but somehow both making tools and surviving on the same foods.
And yet, this "H. habilis" specimen, KNM-ER 42703, is nearly 200,000 years later than any other member of its species. Almost the only things that makes it H. habilis are its third molars. Are they enough? Or is it Homo erectus, too? Is the overlap completely gone, or will this fossil save it?
And what about that little, tiny, H. erectus skull? At 1.6 million years old, KNM-ER 42700 is a part of the earliest African sample. It's 200,000 years younger than Dmanisi. Did they originate in Asia? Did they evolve directly from their immediate predecessors in Africa, the larger habilines?
You see, this is interesting stuff! It's like a Plio-Pleistocene soap opera -- complete with twins separated at birth, old characters being killed in Amazonian plane crashes and mysteriously returning disguised as someone else.