The human population bottleneck

I’ve been reading up on the ‘bottleneck’ of the human species, seemingly around 70,000 years and had a few questions, as the information seems to be somewhat contradictory, so I’d appreciate the wisdom of the millions to clear up my confusion.

Prof. Wikipedia states that after the Toba eruption the total number of human individuals on the planet was around 15,000. In the article on the Toba eruption reduced the population to “…to 10,000 or even a mere 1,000 breeding pairs”

I find in the bottleneck article that an alternate theory is that “…in sub-Saharan Africa numbers could have dropped at times as low as 2,000, for perhaps as long as 100,000 years, before numbers began to expand again in the Late Stone Age[8]”

Some quick questions on this event;

  • Am I right in assuming that every single person after about 70,000 B.C. is descended from one of these ~15,000 individuals concentrated in Africa? There were no ‘pockets’ elsewhere?
  • On the back of this; how much credence is given to the idea that humans, in their infancy, were a few thousand casualties away from extinction? I note that the words ‘suggested’ and ‘postulated’ bandied about a bit, so how sure are we?
  • How close were we to our extinction? How many more casualties would have doomed us?
  • How different as a species would we be if more had survived? Since as a result we’re all, to an extent, quite…well, inbred. More people would have reduced this, I’m guessing.

Thanks for any light shed! Apologies for the many (probably obtuse) questions.

There are theories that put all people descended from the same Male person at 70,000 bce.

So, yes, we’re all descended from the people that lived in 70,000 bce. Its a reasonable conjecture that all the people at that time were still in Africa.

Thanks for the reply; I’ve been reading up on the Y-chromosomal ‘Adam’ which seems to be related, although if I’m honest it confuses me even more. Surely this fella came from somewhere?

It seems correct to say that all the descendants of the other males around at the time snuffed it (or only had daughters who survived), so only his descendants survived, which is where we get the link with him. But what I don’t understand is why we can’t trace his descendants back further.

I hope I’m making at least some semblance of sense here.

You only have one grandfather?

One Great Grandfather?

Nah you have lots of male ancestors.

The other males at the time of Y-chromosome Adam (and their descendants) just haven’t spread their DNA around as fast as Adam and his Descendants. Your ancestors would theoretically include adam and probably many of the males around at his time too.

I’ve no problem with that, but it seems that in order to trace back to 1 individual, a single man, that all the other males around at the same time must have failed to pass on their Y-chromosome (by death or daughter). Otherwise it would seem we would have a load of ‘Adams’;

By analyzing the Y-chromosome DNA from males in all regions of the world, geneticist Spencer Wells has concluded that all humans alive today are patrilinealy descended from a single man who lived in Africa around 60,000 years ago
From the wiki article.

But then I don’t see how we can say one man was the Y-chromosomal Adam - why not his father, grandfather etc? Is this specifically linked to the bottleneck? The timescale seems roughly similar.

I can’t comment on the factual basis for this theory, only the logic.

You have a single line of male ancestry stretching all the way back to this Adam (and beyond, to his father, grandfather, and so back to the beginnings of sexual reproduction). Adam’s father is also a common ancestor, but Adam is the most recent one (in the male line).

Adam had contemporaries, including male contemporaries, many of whom are also our ancestors, but with at least one female link in the line of descent.

I believe, but cannot prove, that any of Adam’s contemporaries who is the ancestor of any living human, is overwhelmingly likely to be the ancestor of all living humans.

Y-Adam is not the only male-line common ancestor, but the most recent one. Obviously his father, father’s father, etc., would also be male-line common ancestors of us all. Likewise, we can say with certainty that Y-Adam had at least two sons, because if he had only one, then that fellow would have been Y-Adam instead.

This also doesn’t mean that none of Y-Adam’s male cohorts had sons: Presumably most of them did. It’s just that all of their male lines died out eventually, not necessarily right away.

And finally, note that Y-Adam is the most recent male line common ancestor, not the most recent common ancestor period. If you take your father’s father’s father’s … father’s father, eventually you get back to Y-Adam, but if you’re allowed to mix the maternal and paternal lines (like, your father’s father’s mother’s father’s mother’s…), then you can find a common ancestor who’s much more recent. It’s estimated that the most recent common ancestor for humans was somewhere between 2000 and 7000 years ago. In fact, if you go back far enough, to the so-called identical ancestors point, you reach a time when every single person alive at that time either has no descendants still alive, or every single person on Earth today is descended from them. Estimates of this point are between 5000 and 15000 years ago.

In addition to Y-Adam, there’s also Mitochondrial Eve, who is the most recent common ancestor in the female line (that is to say, if you take anyone’s mother’s mother’s mother’s … mother’s mother, you get to MitEve). Due presumably to male and female mating habits, MitEve is much further back than Y-Adam, about 150,000 to 250,000 years ago.

It does not relate to any population bottlenecks out of necessity, and I don’t know that it’s reasonable to draw the conclusion that it’s related to a population bottleneck from just the information I’ve read in this thread, without having gone to any of the source papers. All this Y chromosome stuff means, is that if you were to take a DNA sample from all of the men alive some 60,000 years ago, and compare those to a random sample of DNA from men alive today, each and every one of our Y chromosomes will be copied from only a single Y chromosome present in that ancestor population. One of those men is the only one who has had a son in every generation from then to now. For all the others, they either had no children, or they and their sons had only daughters, so that their Y chromosome hit a dead end. They still had children, and many if not all of those men alive 60000 years ago are your direct ancestors, albeit through at least one woman, rather than through your father’s father-to-the-nth-power.

We point to one of those men in the abstract and call him the Y chromosomal Adam because he is the most recent man to have fathered all men alive today. He had more than one son, we know, by definition. If he’d had only one son, that one son would be the one we’ve named Adam instead. Adam’s father may have had more than one son, but we know that Adam’s possible brothers’ Y chromosomes died out, again by definition. If they hadn’t, then we would have called someone in an earlier generation Adam.

On preview, what ** hibernicus** said.

Ah, I think I understand now. He is the most recent of the common ancestors who has been part of the unbroken Y-chromosome chain, the ‘root of the tree’ so to speak? A root which itself stretches back to the origin of life itself; hence why I was a tad confused - it seemed a bit ‘chicken and egg’ like to me.

My understanding is that at the time of the Toba eruption, humans had expanded out of Africa along the south Asian coast as far as southern China. The eruption wiped out everyone living in south Asia, but left some survivors in Africa, southwest Asia and southeast Asia. Within about 5000 years, some of the last group managed to make it to New Guinea and Australia. Note that this was during an ice age and sea levels were so low that most of Indonesia was connected to Asia.

Here’s a fascinating site on the expansion of humans: Journey of Mankind It discusses both the mitochodrial and Y-chromosome dating, since using them can tell when various regions were populated by the ancestors of their current populations. They don’t just use the global mtEve and Y-Adam – various subpopulations have their own mtEves and Y-Adams that tell when, for example, Japan or Europe were settled and who/where their people branched off from.

Fascinating link, thanks dtilque. So the Toba eruption wiped out everyone in the Indian subcontinent (and killed a huge proportion elsewhere) - leaving only two population groups left, the ones in Africa and the ones who were luck and at the ‘other side’ of the eruption in South East Asia. Do we have any idea how the ~15,000 were split between these two locations?

I see from your link that there is genetic evidence that the African group went back across India and recolonised it, mixing somewhat with the SE Asia group down in Australia. Are most of us descended from the African group, which crossed back over the Gates of Grief after the eruption to recolonise India and move on to Europe?