Tuesday, March 16, 2010

on other sub-species, other species, holotypes, the Holocene, and so much more

time for another skull - the reconstructed skull of H sapiens idaltu

Well I've just discovered something quite new, courtesy of Wikipedia. My childhood love of encyclopaedias is being well sustained.

My discovery relates to Homo sapiens sapiens. The third Latinism indicates a sub-species – and another subspecies, Homo sapiens idaltu [completely new to me], now extinct, is 'known' to have existed. Presumably there were others too? I suspect though, what with the possible interbreeding of humans and Neanderthals, that the picture is more complicated still.

Homo sapiens idaltu was first presented to the world only recently, in 2003. The classification is based essentially on the discovery of three crania in Ethiopia, dated to between 160,000 and 154,000 years ago. I'm not sure how controversial this classification is, given the usually paltry fossil evidence relating to early humans, and the relatively large variation in human skulls, but it seems to be generally accepted. Here's what the discoverers of this sub-species have to say about their specimens:

"On the limited available evidence, a subspecies of Homo sapiens distinguished from Holocene anatomically modern humans (Homo sapiens sapiens) by greater craniofacial robusticity, greater anterior–posterior cranial length, and large glenoid-to-occlusal plane distance. Homo sapiens idaltu is distinguished from the holotype of Homo rhodesiensis (Woodward, 1921) by a larger cranial capacity, a more vertical frontal with smaller face, and more marked midfacial topography (for example, canine fossa). We consider the holotypes of H. helmei and H. njarasensis too fragmentary for appropriate comparisons."

For my own sake, let me tease out this paragraph. There's a greater cranio-facial 'robusticity'. I'm not sure if this is a technical term. Does it mean hardness, bone density, bone thickness, or does it have something to do with the shape of the face? Prominent ridges in the eyebrow region, like the Neanderthals [Homo neanderthalensis], though not so marked? I really have no idea.

Greater anterior-posterior cranial length, now that's a bit clearer, that's measurable. Presumably they've provided some detail on that, I'll just take their word for it. Though I do wonder how a greater front-to-back skull length would indicate an earlier species. But I shouldn't of course think of earlier species as less developed. They just died out for some reason. They developed in the wrong way, or just found themselves confronted with the wrong conditions at the wrong time.

The Wikipedia article [I should look into this more] states that the most well-preserved cranium, that of an adult male, had a brain capacity of 1450 cubic centimetres. This article on the evolution of the human brain emphasises the variety to be found in brain capacity of all primates, including humans. The variation among humans is described as between 1100 and 1700 cubic centimetres. It has been averaged out at 1350 cubic centimetres. You would think, maybe, that 'anterior-posterior cranial length' would vary to a similar degree. Just wondering.

Before I go on to the 'glenoid to occlusal plane distance', whatever that means, I need to explore the phrase 'Holocene anatomically modern humans'. Apparently we're talking ultra-modern here, at least in geological terms. The Holocene period covers the period from 12kya to the present [just practising my terminology here].

I could go into much greater detail about the Holocene, its subdivisions, its relation to the Mesolithic age and so forth, but for our purposes its simply the modern era, or the era of human 'civilisation' as generally understood.

So now we turn to the 'large glenoid to occlusal plane distance' of the Homo sapiens idaltu crania. Well, the 'glenoid cavity', which they're presumably referring to, has nothing to do with the cranium. It's part of the scapula which articulates with the humerus. The occlusal plane is 'a plane passing through the occlusal or biting surfaces of the teeth'. Really, I would have to read the detailed finding re this sub-species to get my head around this.

Still, I'll go on. On 'the holotype of Homo rhodesiensis', a holotype is, essentially, the best physical example of a species [or lower taxon], from which the type [the species or other taxon] is taken. Sometimes, as with rare fossils, there may be only one specimen, which may or may not be representative of the type. It's simply all we have. Homo rhodesiensis is a typical example of this, described essentially from one skull found in what is now Zambia [formerly Northern Rhodesia] in 1921. Its dating ranges from 125 to 300 kya. It may well be a specimen of Homo heidelbergensis.

Interestingly, the discoverers of this sub-species make no comparison with Homo heidelbergensis [well they probably do, but not in the cited passage], which presumably, too, has a smaller cranial capacity.

I understand what they mean by a more vertical cranial frontage, though the idea of a more marked midfacial topography isn't entirely clear to me. Well, I suppose it means a bigger nose – though they give the example of canine fossa [fossa is Latin for ditch or trench, so presumably they mean something to do with more entrenched canine teeth, or deep trenches for the canines].

So I'll take their word for it – these remains represent another sub-species, Homo sapiens idaltu, of which we know little except that it existed some 160 kya.

I'm presuming, as I've found no information to the contrary, that the Skhul remains are of Homo sapiens sapiens, though not directly ancestral to us. I'm not sure how that works, actually. So for how long did Homo sapiens idaltu survive, and did it interbreed with Homo sapiens sapiens? All is in thrilling confusion.

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