But let’s keep calm and start from the beginning…
Your DNA is much more like mine than it should be. There should be variation within a species – there should be because it helps any species survive under challenging evolutionary times, but anyway we should expect a lot of variation in a species with our huge population. Obviously in a bigger population you’d expect more diversity, especially if it had been big a long time. But there’s only as much variation in our 7 billion people as you’d expect in a standard/ideal species with a population of 50,000. Our “Effective Population Size”, as the geneticists say, is about 50,000. That’s smaller than the actual population of chimpanzees. It’s even smaller than their effective population size which is almost always quite a bit smaller than the actual population, for any species.
The old reason was that the Toba volcano in Sumatra erupted 75,000 years ago. Its eruption is undisputed, but the effect on the human genome is not universally accepted (just googled it – Oh! That person started it 🙂 . I might have done at that time if I’d thought of it.) If the eruption almost wiped out the human species, we’d obviously only have a small population at that time, and you can’t hold much genetic diversity in a small population. But we’ll come back to all this and other juicy tangents later. First though…
I can honestly say that since I started using eZvol regularly, my life has changed. Or at least I’ve got much more into genetics. It’s taught me a bit, confirmed a bit, and disproved a bit, but I’ve also started reading a lot of stuff. Anyway I can now say with certainty (as I hope I always could) that evolution stops only when it’s working hard, and then only sometimes. It’s pretty obvious really, that if most of your DNA needs to be exactly as it is for you to survive, then almost any mutations will be “ironed out” of your descendants by evolution, within a very few generations. Evolution will be working hard to keep things the same.
On the other hand, if you have a lot of “junk DNA” (glance over your shoulder before you use that term!), then any old change can happen in it, and stay. The DNA we still have in unused decayed form, that helped us twitch our ears like a cat, can change all it likes and it won’t harm our survival chances or those of our offspring. (A little function remains though – nerve impulses are sent towards our ears but not much happens. They use that to see if people are deaf.) So even if the non-junk DNA – the DNA that actually does something – has its brakes firmly on because you’re already the Best A Man Can Get (your gender and age may vary), your junk DNA will still change randomly and freely. In fact if the mutation rate of your junk DNA is one per generation, then after 1,000 generations, you will have about 1,000 new mutations in it (unless your genome is so small that new mutations start going in on top of old ones.) This even applies to the whole gene pool, even if every individual in the population gets just one mutation. The overall clock for the whole population is still one per year. It occured to me while reading King J L & Jukes I W 1969 “Non-Darwinian evolution”, Science 164:788-98, over the weekend that this isn’t actually as obvious as it sounds when it comes to fixation, but it is true. I trust eZvol more than any geneticist now, but the computer says yes, so it’s yes.
What though, when even your non-junk DNA starts behaving like junk because in the mopdern world no matter how unhealthy you are (or allow yourself to become), you can still raise children? One of my favourite examples is traction. Before 1917 when the Thomas procedure was invented, most died if they broke their femur, but now few do. But importantly, in the modern world this also applies to genetic mutations. Because mutations don’t get swept out now, even those that destroyed our ability to chase down a mammoth and bring it home to our freezing kids so we could make a fire from its bones and use its hide and tusks for a house, Mutations Just Stay. And Build Up. Which means our genetic clock and our genetic variation build up faster when the pressure’s off.
Steve Jones said in “Word of Mouth — Language and Our Genes with Dr Steve Jones Michael Rosen talks to Steve Jones about language and our genes, 23rd Oct. 2017, BBC Radio 4”, that since we adapt more using cultural evolution now, genetic evolution does less, which is why we have low genetic diversity. I just shouted “Oh, GOD!!”. I don’t know what the neighbours thought. It was nearly midnight. Behind his statement is the implication that he knows less about genetics than me. And that’s not good news because he was a professor of genetics at a top university. And of course it means that the BBC’s second favourite genetics expert/pundit knows less about genetics than a well-trained third-year genetics undergraduate, since such a person should know more than me.
Interestingly… most genetics professionals would find Jones’ opinion more shocking than I do since they don’t realise as I do that under some circumstances, this time when evolution is working very hard to adapt to changing environments, the genetic algorithm can actually drive the genetic clock faster than the background mutation rate. John Hawks doesn’t think that it can, and he’s already declared himself against the Jones view that human evolution has slowed (or even stopped; of course JH is actually an anthropologist but it’s true for most geneticists too). I think Jones must be taking the view that when a species needs to change, mutations can be selected for and multiplied throughout the population, driven hard by the forces of evolution… and that without evolutionary force, everything slows down. He’s wrong about the last bit but slightly right about the first bit: under strong evolutionary pressure and relatively low mutation rates, things can speed up – faster than the background mutation rate. It’s complicated by the fact that since most of our genome is junk, genetic drift in the junk often or usually overpowers the rate of change in the non-junk – and the genetic clock in the non-junk isn’t always much faster than in the junk. Also, surprisingly, genetic drift in the NON-junk has quite a considerable effect. I’ve never been keen on the idea of genetic drift, but the more you read, the more you realise its importance… and yet non-drift “evolution-driven” genetic change is still more significant than the average modern geneticist realises. But that’s another story.
What then did cause the low genetic diversity in humans? The story now told is that when modern non-Africans did their Moses thing about 65-70 thousand years ago and crossed the Red Sea into Arabia and beyond (by ‘boat’, though the southern end was narrow since much of the sea’s water was locked up in ice), it was a relatively small group. Perhaps very small. That’s why they didn’t bring much diversity with them. But even though they interbred with the Neanderthals they met in Arabia, and some interbred with the Neanderthals’ near relatives the Denisovans before wiping them out, and some interbred with another group in East Asia where people there still have bucket-seat-shaped incisors they probably got in the process, they still had much less variation than exists in Africa to this day. But of course that doesn’t explain why Africa itself has very low diversity.
The reason is this. The Out Of Africa story is only largely true. Most of our evolution did occur in Africa. It’s now accepted that waves of people long before the final eruption, repeatedly left Africa for the rest of the world, and then got impacted by later waves. But humans didn’t necessarily make the final leap to Homo in Africa. There is every reason to believe the mix of early Homo and pre-Homo fossils Lordkipanidze and others found in Dmanisi in Georgia are better evidence for a transition than anything known from Africa – or perhaps will ever be known, if the transition was indeed outside Africa. Incidentally, the physical variation in the Georgia fossils was large, suggesting to me that Georgia was just a place where physically diverse groups met – not necessarily the centre of anything.
In that case, there will have been a restricted population having left Africa as pre-Homo about 2 million years ago, restricting the diversity, and then a re-invasion back into Africa, maybe as Homo ergaster, shortly afterwards, restricting it again.
It’s also possible that modern humans didn’t exterminate all the non-moderns in Africa as efficiently as we seem to have done elsewhere – the Denisovans may be an example. We didn’t even exterminate or dilute the Neanderthals as quickly as we might for perhaps two reasons: first, the Neanderthals weren’t a push-over. They were much like us, but perhaps a little less keen on tolerating or trying to meet people they didn’t know. That’s in shorthand but it can be defended rigorously. Also, it’s been suggested that they weren’t very interfertile with us, in much the same way that dogs and wolves don’t interbreed anything like as efficiently as they do amongst their own types, as you would know if you’d tried. There was interbreeding but less than there might have been. But the survival of Homo naledi in the heart(-ish) of Africa, and according to rumour, of other types in West Africa who donated genes to moderns, suggests that for say half a million years at a time, at least, we co-existed somewhere in Africa with others, whereas it only took 40,000 years to remove Neanderthals from Europe. Maybe there were types of locations in Africa we prefered to leave to others.