an autodidact meets a dilettante…

A memory keeps flashing back to me of an argument I was having with a Christian friend. I said something about humans being around on this planet for x number of years. No doubt I was making the point that if Christianity has only been around for 2000 years, why did their god not allow all those earlier humans to ‘know Christ’ and presumably therefore be saved? I’m hazy about the exact nature of the argument because we were both quite drunk at the time. Anyway, his response to my educated guess about the length of time humans have been around was ‘bullshit – that’s complete bullshit’. I was taken aback – it’s not as if my friend was a young earth creationist or anything, he just thought my guesstimate was way out. Trouble is, I can’t recall whether I’d said we’d been around for 200,000 years or 500,000 years or a million years or two million. How would I know?

So I’m going to have a go at answering this question here, but before I do I might just raise an obvious issue which may or may not be philosophical. If we decide, based on fossil or genetic evidence, that the first humans lived, say, 550,000 years ago, then what about the parents of those humans? Were they suddenly not human? Not likely. There would’ve been essentially no observable difference between the ‘first human’ and its parents, and then again no observable difference between those parents and their parents, and so on back down the line. So how can the human possibly begin at all? Can’t the human be traced back to the first living thing, which would therefore also be human, in some real sense?

This is surely more than a piece of pedantry. It points up the artificiality of species differentiation, when in fact everything and nothing can be differentiated. You could counter by saying that the differentiation becomes important when it has some significance, but that would be circular. What constitutes significance?

So maybe we can’t ultimately answer this question, but we can surely try for some sort of pragmatic answer. After all, we don’t categorise Homo sapiens as separate from Homo erectus for nothing, do we?

One approach used is the Adam and Eve approach, that’s to say mitochondrial Eve and Y-chromosomal Adam (I’m not sure about these designations, which play into certain hands – after all, in another, safely dead, tradition, the first woman was Pandora, another symbol of the evil woman] . Mitochondrial Eve is our matrilineal MRCA [most recent common ancestor]. I’ve already explained, in the recent post on Neanderthals, how mitochondrial DNA, as opposed to nuclear DNA, is passed down the female line without the recombination that is the product of meiosis or eukaryotic reproduction. In fact, due to its protection within the ovum it doesn’t undergo recombination at all. It follows that we can trace our mitochondrial DNA back to an MRCA, estimated at 200,000 years ago.

But MRCA doesn’t by any means equate to the first female human. This ‘Eve’ had plenty of contemporaries – sisters, cousins, parents and grandparents, who were obviously just as human as ourselves, it’s just that their descendents aren’t traceable to modern humans in an unbroken line. The story of Y-chromosomal Adam [or Adams] is quite a bit more complicated, and considerably less resolved, and I won’t tell it here. Suffice to say that, whether or not our male MRCA turns out to be older than mitochondrial Eve [our current candidate is much younger], the same problem arises – his parents and his great-grandparents will seem, to all intents and purposes, to be just as human.

So let’s leave genetics aside and turn again to the fossil record [I’ve written quite a bit on this already, e.g here, here, here, here, here and here]. As everyone should know, this record is scratchy and controversial. One of the problems is that, given normal human variation in cranial capacity and other measures, it’s often very difficult to distinguish between a fossil of [part of] a Homo sapiens and another Homo species, of which we may have only one or two [partial] specimens with which to make comparisons. We may with varying degrees of confidence claim that Homo sapiens evolved between 400,000 and 250,000 years, but it wouldn’t be unreasonable, surely, to claim that the species of Homo that preceded Homo sapiens was also recognisably human. We know now, for example, that Homo neanderthalensis was human enough to successfully breed with.

H erectus, a species believed by many to be ancestral to H sapiens, was generally thought to have died out about 400,000 years ago, but recent work on specimens found in Java in the thirties, have led them to be reclassified [always tentatively and controversially] as a sub-species of H erectus, dating back only 50,000 years, or less. And then there’s the recent H floresiensis, which may yet be reclassified. The extinct species H heidelbergensis, which flourished in Europe between 400,000 and 600,000 years ago, is also a candidate for our immediate ancestor, though it’s disputed whether H heidelbergensis is really a separate species of H erectus or H ergaster [whose existence as a separate species is also in doubt].

So are we any closer to answering our question? My uneducated guess is that all of these species and sub-species of Homo [and there are many more than I’ve mentioned here] are variants on a line of descent dating back two million years or so, and that terms such as ‘species’ here are often more misleading than helpful. This brings me to the ‘multiregional hypothesis’ of human descent, summarised in Wikipedia thus:

The hypothesis holds that humans first arose near the beginning of the Pleistocene two million years ago and subsequent human evolution has been within a single, continuous human species. This species encompasses archaic human forms such as Homo erectus and Neanderthals as well as modern forms, and evolved worldwide to the diverse populations of modern Homo sapiens sapiens. The theory contends that humans evolve through a combination of adaptation within various regions of the world and gene flow between those regions. Proponents of multiregional origin point to fossil and genomic evidence as support for their hypothesis.

All of which sounds more than plausible to me, especially as it allows me to say that humans have been on this planet, in some form or another, for nigh on two million years. It’s a particularly useful stat for baiting the religious – two million years and Jesus drops in only 2 thousand years ago. What about those poor lost souls scratching around for a million nine hundred and ninety-eight thousand years before that?

Anyway, it’s an intriguing story, so let’s keep on digging and researching to fill in the gaps or to create new conundrums for ourselves about ourselves.