The Lucy Show
To the extent that I can claim to be a qualified anything, that would be a Physicist. I am not a biologist, geneticist, anthropologist, or theologist for that matter. But it doesn’t stop my mind from wandering into these areas from time to time. And recently, I have been thinking a bit about evolution.
While working out on my cross-trainer I like to watch a TV show to alleviate the boredom. Preferably something that will interest me sufficiently to extend my workout to the point where it actually does me some good. Recently, I saw a show that made some unconvincing point or other about “Lucy” – the one who was supposedly an Australopithecus Afarensis who lived some 3-odd million years ago. The gist of the program seemed to be founded on the notion that Lucy was a common ancestor of all humanity, and that we are therefore all her direct descendants. It is all speculation, of course, but it got me to thinking about what it means to be descended from someone or something. Because, unless you buy into some sort of creation theory, we all, ultimately, have to be descendants of something that first appeared in the primordial ooze. And I got to thinking about that.
The first thing that strikes me is the notion of being descended from someone. Usually, long-chain blood lines descend down from a given person, and not up to him or her. This is because the historic record is light on the general, and heavy on the specific. So, if you want to trace your ancestry, you probably won’t have to go back very far before you’ll strike out, with apparently no traces remaining of any records regarding certain individuals. In my case, my family tree peters out after only three or four generations. Perhaps not a bad thing, I sometimes think.
But one thing we can be pretty sure of, and that is that every human being who ever lived had one extant mother and one extant father (OK, all but one, if that is a point you want to argue). So, starting with myself, I can say with certainly that I had two parents, four grandparents, eight great-grandparents, sixteen great-great grandparents, and so on. I may never know who they all were, but I know they did exist.
You can imagine a hypothetical map of the world on your computer screen, with a slider that controls the date going back as far in history as you want. As you move the slider, a pixel lights up showing the whereabouts of every one of your direct ancestors who was alive on the corresponding date. If such a thing could ever exist, I wonder what it would show. My father is a Scot and my mother Austrian, so I imagine that for the first few hundred years or so mostly Scotland and Austria would be lit up.
For all of my most recent ancestors, it is a fair assumption to suggest that they are all mutually exclusive. In other words, that there was no cross-pollination (if I may put it that way) no matter how far removed the individuals were in my family tree. Realistically, though, that approximation is going to falter if you include a sufficient number of generations. Therefore, as we go further back in time the net number of my ancestors stops growing at an exponential rate. But at the same time as the number of my ancestors has been growing, with the steady rolling back of the clock so also the total number of humans on the planet will be shrinking. The growing ancestral base, and the shrinking overall population must surely meet somewhere.
It seems reasonable that the same line of logic should apply to us all. In other words, if we go back far enough in time, we should find that we are all descended from the same group of humans, no matter how disparate geographically, culturally, or any-other-ally. But this group would not comprise all of humanity at that point in time. Some of those individuals will die childless. Others will bear children who will die childless, and so forth. So the entire human race at that point will comprise two groups of people. Those who are the direct ancestors of every living person in the world today, and those whose bloodlines died out completely in the intervening millennia.
So the questions that I arrived at were these. What would the expected ratio be of ancestors to non-ancestors? Would we expect it to be a relatively large percentage or a relatively small percentage? And in particular, if the latter, what would it take for that small percentage to actually be one person? Is that even possible? I have never seen this line of thinking expanded upon, but one thing I have learned is that whenever something like that crosses my mind, it has always previously crossed the mind of someone who is a proper expert in the field. Maybe one day I’ll get to hear that expert’s opinion. But, in the meantime, it seems highly improbable to me that the ancestral percentage would even be a minority, let alone a minority of one.
The next point concerns what you might term the crossing of the man-ape barrier. This troubles a lot of people. Scientists dig up ancient skeletons and fossils and assign them to categories such as human, proto-human, and ape. Actually, they are lot more scientific about it, but you get my drift. The theory of evolution provides a mechanism or road-map for the development of ape into proto-human and proto-human into human, but has little to say on the specifics. Meanwhile, all we have in our historical record are a seriously limited number of archaeological specimens that we can do little with other than to fit them into a timeline.
The transformation of proto-human into human took – I don’t know – let’s call it a million years. Yet we only have specific archaeological specimens – for example the proto-human and the human. Us ordinary folk look at them – and also at the artists renderings of what the original individuals may have looked like – and many people have a hard time grasping how it is at all possible for one to become the other. Of course, if we had a perfect fossil record – say, one for every thousand years over the span of that million years – we might be able to understand and communicate convincingly how the development played out. But we don’t. And so we can’t. We can just make guesses – albeit highly-informed and very well-educated guesses.
These things happened over timescales so vast that all of recorded history is just a blink right at the end. It is wrong to think of evolution as a set of stable eras characterized by specific inhabitant species, separated by periods of transition. Certainly major transformative periods did occur, such as the Jurassic/Triassic, Triassic/Cretaceous, and Cretaceous/Tertiary (K/T) boundaries, but in general, for the last 66 million years, evolution has been a continuous thing. We are evolving today as a species as least as quickly as – if not orders of magnitude faster than – our ancestors did as they transitioned from proto-human to human. We’ve just not been around for long enough to be able to observe it.
Lets close with my computerized ancestral map, and slide the time dial back to the age when proto-humans were evolving into humans. Assuming that all humanity does not derive from The Lucy Show after all, my ancestral map will become a map of proto-human occupation. Slide it back a bit more and it will reflect ape occupation. Slide it back even further – and then what? At that point as far as I can tell all science has to offer is speculative at best. Apes date back to the cretaceous period. So, although we pretty much certainly were not descended from dinosaurs, it seems likely that some of our ancestors will have been eaten by them (although if a T-Rex ate some of my relatives, it would perish from alcohol poisoning). On the other hand, the well-known Dimetrodon – a lizard-like creature characterized by a spiny sail along its back, and recognized by five-year-olds everywhere – is quite possibly an ancestor of today’s mammals.
At the far end of its travel, my ancestral map ends up in the primordial soup, presumably as a population of bacteria. But if it did, then so did yours!…