| A very modern ape? |
Just for the hell of it, I´ve been trying to
read and comprehend two scientific papers lately: “The Changing Face of Genus
Homo” by Bernard Wood and Mark Collard (published in 1999) and “From
Australopithecus to Homo: The transition that wasn´t” by William H Kimbel and
Brian Villmoare (published in 2016). I found them hard to understand and even
harder to explain to the readers of this blog, who are of course eager to learn
all the cladistic aspects of the australopith-to-Homo transition (when not busy
trying to comprehend the present economic crisis or cold spell). Or am I just
being vaguely ironic here? But sure, I might have gotten *some* kind of existential
insights even from these papers.
I long wondered why the genus Homo (“humans”) is
so broad. As a layperson, I always suspected that a number of taxa in said
genus aren´t really “human” at all, but rather australopithecines. More
specifically: Homo habilis, rudolfensis, floresiensis, luzonensis and naledi. The
first actual human would then be Homo erectus (or Homo ergaster). I always got
the impression that there was some kind of non-trivial “jump” from earlier
forms to Homo erectus. And indeed, both H. habilis and H. rudolfensis have been
classified as Australopithecus by some scientists. (In colloquial terms, the
australopithecines would be “ape-men” rather than real men!) In their 1999
paper, apparently still something of a classic, Wood and Collard argues for
precisely this kind of position: Homo habilis and his close cousin Homo
rudolfensis really were relevantly different from Homo erectus and should therefore
be excluded from the genus Homo. They tentatively reassign these two forms to
Australopithecus, but don´t rule out that they should perhaps be classified in
a genus all their own.
The 2016 paper, by contrast, takes the exact
opposite position: there simply isn´t any dramatic shift from a generalized form of Australopithecus to
an advanced and specialized Homo with large brain, tool-use, obligate
bipedalism, or whatever it is that supposedly makes us human (all too human). Nor
is there a smooth gradual transition from one to the other either. No,
evolution is like a bush with many branches, the traits we consider extremely
important showing up helter skelter (from our perspective) in quite different
lineages. Of course, this has been the official view of human evolution for a long
time (“three decades” according to the authors), so who needed to hear it in
2016? Or today? Wood and Collard, perhaps?
Kimbel and Villmoare point out that Homo erectus
and its supposed ancestor Homo habilis actually show up at approximately the
same time in the fossil record. I admit I had no idea! It also turns out that
in terms of brain size, Homo erectus, H. habilis and Australopithecus (“sensu
lato”) partially overlap. The smallest brain size of Homo erectus overlaps with
the largest brain size of Australopithecus (!), while the smallest brain size
of Homo habilis overlaps with the middle range of Australopithecus (I´m almost
tempted to say “the midwit range”). This obviously means that the lower range
of Homo erectus also overlaps with that of Homo habilis. The authors believe
that the earliest fossil remains clearly diagnosable as Homo are closer to Homo
erectus (!!) than to Homo habilis, but also that other almost equally old fossils
are more habiline in nature, suggesting that there was a “bushy” diversity in
the human family tree already from the start.
Why haven´t we been told this before? In
popularized docus, I mean.
There are other relevant overlaps too, such as
the “robust” australopithecine Australopithecus boisei having the same body mass
as Early Pleistocene Homo. One thing that was news to me is that nobody really
knows who/what developed the Oldowan industry. I assumed that it was well-established
that these ancient stone tools were manufactured by Australopithecus, but they might
as well have been made by Homo. Who knows, maybe they were made by both?
In the background lurks the conflicts over
cladistics, which co-exists in an uneasy relationship with the old more Linnean
(or perhaps quasi-Linnean) system of taxonomy. Back in 1999, Wood and Collard believed
that cladistic analyses couldn´t distinguish between ancestors and sister groups,
making it impossible to define a genus in a “monophyletic” way (only including
species descended from a common ancestor, plus that ancestor). Instead, a genus
has to be defined as an “adaptive grade”, which I presume means on the basis of
morphology and/or behavior, species having the same “ecological” strategy being
grouped together, the assumption presumably being that this does express common
evolutionary ancestry. Indeed, Wood and Collard believed that a close analysis
of hominin fossils shows that Homo habilis and H. rudolfensis had an adaptive
strategy relevantly different from those of, say, Homo erectus. This would have
aligned them more closely with Australopithecus.
Kimbel and Villmoare by contrast believe that
such a dramatic rift doesn´t exist. *If* there is a transition between
different adaptive strategies, it´s visible in the oldest known remains that
could be Homo, remains far older than Homo habilis. Of course, the 2016 authors
believe that cladistics work. The Linnean categories, by contrast, are
meaningless and arbitrary. There really is no such thing as a “genus”, and the
same is true of “family”, “order”, etc. Asking how to define their limits is
like saying “how far up is up” (ironically, Wood and Collard have almost
exactly the same criticism of cladistically-based genera!). What´s important is
the concrete evolutionary lineage, something the 2016 paper believes can be demonstrated
by cladistic analysis. Thus, scientists can objectively construct monophyletic groups
(clades). I´m not sure how they do this with fossils, however, since no DNA can
be extracted from them? Some kind of advanced computer modelling?
There are of course “ideological” implications
of all this, or at least of Kimbel´s and Villmoare´s paper, something the two
authors also emphasize. If human evolution was neither a dramatic shift from Southern
Ape to Man, nor a smooth, unilinear and progressive transition between them, Homo
sapiens isn´t unique. We like to imagine our species as towering on the endpoint
of evolution, with our suit of characteristics defining whatever “transition”
we think lead to us. In other words, a kind of hidden teleology. Instead, the transition
was complex, with each of our “important” and “defining” characteristics
presumably appearing at different times, perhaps originally in different groups
of hominins. We are simply the lucky recombination of all these characteristics
(that´s my interpretation) imagining ourselves to be the unique goal of the
process. The 2016 paper in effect calls us to go back to Darwin and more
radically accept the implications of his theory.
So perhaps the “genus Homo” really is broad enough to include even Homo habilis & Co.
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