The skein of human migrations out of Africa is quite tangled. We of course evolved in Africa, splitting off from the lineage leading to bonobos and chimps about 5-6 million years ago (mya). The first Homo foray out of Africa was probably Homo erectus, which might have left about 1.75 mya and then spread all the way to eastern Asia by 1.5 mya. Then they died off for reasons unknown. (All of this is tentative and subject to revision after future research.)
Our ancestors also split off from a lineage destined to leave again, at various times estimated from 500,000 years ago to 200,000 years ago. That lineage split into the sister subspecies Neanderthals and Denisovans (I consider them subspecies of Homo sapiens), and perhaps into the tiny species H. floresiensis, which lived on the Indonesian island of Flores (dating is wonky here).
The conventional wisdom is that all of these subspecies and species went extinct until “modern” Homo sapiens made its Big Exit into Eurasia about 50,000-60,000 years ago, proceeding to colonize the world. Now, as Carl Zimmer reports in the NYT (click headline below or find article archived here) there’s increasing evidence that modern H. sapiens might have left Africa a lot longer ago: about 250,000 years ago. That’s a substantial revision of our migration out of Africa.
I’ve indented excerpts from the article:
Several new studies, including one published on Thursday, argue that the timeline was wrong. According to new data, several waves of modern humans began leaving the continent about 250,000 years ago.
“It wasn’t a single out-of-Africa migration,” said Sarah Tishkoff, a geneticist at the University of Pennsylvania. “There have been lots of migrations out of Africa at different time periods.”
Those earlier migrations went largely overlooked until now, Dr. Tishkoff said, because the people who moved did not leave a clear fossil record of their existence, nor did living people inherit their DNA.
And here’s the evidence that modern H. sapiens left Africa a lot earlier than we think:
Dr. Paabo’s team also discovered that living, non-African people carry fragments of Neanderthal DNA, a signature of interbreeding from long ago. In May, a team of researchers estimated that Neanderthals and modern humans interbred during a short period of time, between 47,000 and 40,000 years ago.
But some Neanderthal DNA does not fit into this neat picture. The Neanderthal Y chromosome, for example, is more similar to the Y chromosome found in living humans than it is to the rest of the Neanderthal genome.
In 2020, researchers offered an explanation: Neanderthal males inherited a new Y chromosome from humans between 370,000 and 100,000 years ago. But that would have made sense only if a wave of Africans had expanded out of the continent much earlier than scientists had thought.
Researchers have recently found evidence for such an early wave in the genomes of living Africans.
Dr. Tishkoff and her colleagues compared the genome of a 122,000-year-old Neanderthal fossil with the genomes of 180 people from 12 populations across Africa. Previous studies had found no sign of Neanderthal DNA in African genomes. But Dr. Tishkoff’s group detected tiny pieces of Neanderthal-like DNA scattered across all 12 of the populations they studied.
When they examined the size and sequence of those genetic fragments, they concluded that Neanderthals inherited them from early Africans. That meant an early wave of Africans expanded into Europe or Asia about 250,000 years ago and interbred with Neanderthals.
This conclusion depends critically not just on the dating of the Y chromosome and other bits of DNA, but also on the date of the migration of the Neanderthal/Denisovan lineage out of Africa. If, for example, the Neanderthal lineage had exchanged genes with the modern H. sapiens lineage in Africa between 370,000 and 250,000 years ago, and THEN the Neanderthal lineage migrated to Europe, we wouldn’t need to invoke an earlier migration of modern humans out of Africa. I trust that the dating of the Neanderthal migration out of Africa (600,000 years ago or so) is sufficiently accurate that the scenario I invoked wouldn’t have happened. But as far as I can see, the date of Neanderthal migration out of Africa is contested. I’ll punt and take the attitude that “Popppa knows best” since Tischkoff and Paabo are both excellent researchers.
There’s also another study suggesting early migration out of Africa:
Another group of researchers — led by Joshua Akey, a professor of genomics at Princeton University — tackled the same question with its own statistical method. After comparing the genomes of 2,000 people from across the world with three Neanderthal genomes, they reached the same conclusion.
As Dr. Akey and his colleagues reported on Thursday, modern humans expanded out of Africa and interbred with Neanderthals between 200,000 and 250,000 years ago.
But Dr. Akey’s team also found evidence for yet another early wave. By comparing the genomes of young and old Neanderthal fossils, they concluded that another group of people migrated from Africa between 120,000 and 100,000 years ago.
As Steve Gould once said, he always prepared for his class on human evolution by throwing away all his notes from the previous year’s lecture and rewriting his spiel. This is how fast things change, particularly now that Paabo and colleagues pioneered the study of hominin fossil DNA.
One question remains: if modern H. sapiens really did leave Africa between 370,000 and 100,000 years ago, what happened to them? One thing we do know for sure from copious DNA and skeletal and artifact dating is that all modern humans descended from a group of ancestors that left Africa around 60,000 years ago. There’s very little doubt about that.
This means that those earlier migrants didn’t leave descendants; they went extinct and are ex-hominins, singing with the choir invisible. What happened? The article suggests that “African populations built up cultural knowledge that led them to make new inventions, like arrows, and adapt to new places more successfully.” The older H. sapiens then would have been outcompeted or even killed off by the new arrivals. As usual, we don’t know, nor do we know why the Neanderthals and Denisovans (or, for that matter Homo erectus) went extinct.
It’s a good thing I’ve stopped teaching my lecture on human evolution (I got only 1.5 hours on this in my short Evolution segment), as I’d have trouble keeping up with these changes. There are few human remains and dating is imperfect, so what’s sure to happen is that the story above is likely to be revised—except for the part that all living humans are brothers and sisters who evolved from a band of ancestors who left Africa a few tens of thousands of years ago.
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Here’s a “classic” Neanderthal skull from Wikipedia, labeled this way:
La Chapelle-aux-Saints 1 (“The Old Man”) is an almost-complete male Neanderthal skeleton discovered in La Chapelle-aux-Saints, France by A. and J. Bouyssonie, and L. Bardon in 1908. The individual was about 40 years of age at the time of his death. He was in bad health, having lost most of his teeth and suffering from bone resorption in the mandible and advanced arthritis.
Neanderthals didn’t live very long. Poor guy!
“One thing we do know for sure from copious DNA and skeletal and artifact dating is that all modern humans descended from a group of ancestors that left Africa around 60,000 years ago.”
Does this include modern African populations? In other words, did some descendants of the group who left Africa 60,000 years ago come back to Africa, such that they are the ancestors of modern Africans? Or are the ancestors of modern Africans people of the same genetic stock who stayed behind and never left Africa?
I don’t understand that sentence either, Prof. Coyne?
” (I consider them subspecies of Homo sapiens),”
I sort of think of them as diff species but – and this is amusing – you ACTUALLY WROTE THE BOOK! on speciation, so I’ll defer to your much better judgement.
I’m not picking any fights with the Terminator, not trying to match my skill as a trader with Soros or my acumen as an attorney with Dershowitz!
🙂
An aside, few topics have become more interesting in the past decades as all these new discoveries in our history/biology.
D.A.
NYC
Since it’s been shown that we (non-sub-Saharan Africans) have some Neanderthal DNA in us, our ancestors must have bred with Neanderthals and produced fertile offspring. By the “Biological Species Definition” that makes our ancestors and Neanderthals the same species.
I wonder about the sizes of the migrations and the rate of population growth in Africa. If the early migrant groups were small, they would have had a smaller chance of surviving.
I read people arguing about it online and trying to explain certain genetic distribution patterns 15 years ago. They said that SW Asia (Arabia et al.) was already part of the territory of modern humans at least since 130 000 years ago and that this region was the stepping stone for the later bigger expansion. Others said these were failed attempts of expansion. There was no DNA data like this, but there were archeological evidence suggesting earlier modern human presence in Asia. This article brings substantial new information, but the results are not completely unexpected.
All very cool, but why wouldn’t we also just assume that there were several migrations of Neanderthals out of Africa??? Couldn’t that explain some of this?
That would assume there was a long lasting population of them in Africa, but as far as I know no Neanderthal remains were found there. They evolved from a population that left earlier and the part of their African source population that stayed home partly went extinct, partly became the ancestor of modern humans.
Edit: There was article about this that contains the timeline and some data about gene flow.
https://whyevolutionistrue.com/2024/02/26/gene-flow-from-neanderthals-and-denisovans-to-modern-humans-and-vice-versa/
No several migrations of Neanderthals out of Africa couldn’t explain finding bits of older modern human DNA in Neanderthal genomes. If those bits were African Neanderthals, they would coalesce (join together in the genealogy or the phylogeny) with other Neanderthal genes from Asia at an older time. They would just push back the age of the Neanderthal plus Denisovan lineage. What the new data show is that *younger* bits of DNA from Neanderthal bones coalesce with modern human genomes (not with Neanderthal genomes) at an intermediate time *after* modern humans split from Neanderthals but *before* the 60,000-years-ago migration of the later modern humans from whom all the rest of us non-Africans are descended. Sorry that’s sort of complicated. As Jesse Singal says, it’s complicated.
Curious if migrations out of Africa correspond with the 20K year greening cycle of the Sahara and Arabia.
It seems unlikely to me that an older branch of homo sapiens could have been completely killed off by a younger branch for the same reason one tribe is seldom completely wiped out by another. “We have killed off every member of the enemy, permanently wiping their presence from the land — stealing their goods, eating their crops, and taking their women for ourselves! No trace remains!”
They don’t just fight.
This is very compatible with (but not required) for the transfer of genes between “anatomically modern” (or “Cro Magnon”) humans and the Neanderthal (and Denisovan) lineages.
When you think how lucky we are to find any traces at all of people who lived scores to hundreds of thousands of years ago and migrated all over the earth, it’s not surprising that each new find and each technical advance shines light into a previously dark corner.
I’m grateful that you can curate these discoveries for us.
Very good. I can make notes of it, but I doubt I can include it in my evolution lectures. I think I manage about 3 hrs on the subject (starting with the origin of Primates, really), and I leave so much cool stuff out!
The neanderthal Y chromosome detail makes sense to me, since the Y is essentially a non-recombinant chromosome, preserving DNA from a single paternal line without mixing with other Y chromosome DNA.
I like that you describe how earlier (meaning less technological) populations were presumably outcompeted and let’s be honest, often murdered by later (more technological populations). I’ve gotten push-back for even suggesting this uncomfortable interpretation, but it sure seems like that is what we do!
This is a non-sequitur, but it is sad that after all these tens and hundreds of thousands of years, we still have not learned how to live together to maximize the health of our planet and our species. Our monkey nature becomes more evident with each passing day.
Collective-action problems become intractable once you lose an all-powerful sovereign who can enforce the same rules on the entire polity. That can happen once the first small strait of deep water or mountain range is crossed by one arm of a migrating population who then crowns its own king.
It has long been a presumption among leftist thinkers that “cultural knowledge” enabled us to out compete earlier migrants. Even though our brains aren’t any bigger, with our new fangled spearpoints we sure showed those uncouth ruffians a thing or two!
How hubristic/racist can you get? Did all those widespread others disappear because we fought with them? Ate up all their mammoths? Overcame them with our fecundity? There weren’t enough of us to do any of that!
Instead, perhaps those who stayed behind in Africa until 60,000 ybp co-evolved with a novel zoonosis to which our old world cousins had no previous exposure. Many of today’s worst human diseases are zoonoses that emerged from Africa. Marching north worked for “us”, as it later did when the “new world” was discovered. I would further conjecture that much of the Pleistocene megafaunal extinction spasm was due to the long range migration of plaguey humans and their rabid mangy flea ridden semi domesticated wolf companions.
Fascinating! In my History of the Earth and its Life course, I gave one lecture annually on human evolution. Yep. It changed every year. I don’t keep up as closely now, but I am truly amazed at how much we’ve learned over the past 30 years. It’s astounding.
There are two prehistoric sites in Israel in which burials of archaic Homo Sapiens were found: Kafzeh cave (85,000-100,000 years ago) and Skhul cave (over 100,000 years ago). So, we knew at least some HS left Africa before 60,000 YA (fir the uninitiated, Israel is in Asia, albeit right next to Africa). I’m glad for the new genetic information though, it’s always nice when things come together!
Yes, I know about these but they apparently didn’t leave modern descendants.
“… if modern H. sapiens really did leave Africa between 370,000 and 100,000 years ago, what happened to them?”
So seems they interbred with Neanderthals, hence how we now know of them.
But why did they not breed with our “modern” [c60kya] h. sapiens?
Leave more evidence of them?
Unless… were extinct before “we” arrived?
A couple of data points that Zimmer seems to have forgotten about – or chosen to exclude, for some reason.
Unless there has been a substantial re-dating and re-analysis of the fossil deposits at Dimanisi in Georgia (uphill form Tbilisi, but not in the high Caucasus), there is a well-characterised Homo erectuspopulation there, at the junction of the “silk road” and “Europe” and “southern Asia” at about 2 Ma BP. Whether they had descendents has been a matter of controversy for 20+ (30+?) years now.
This sits quite uncomfortably with the reports of hominind footprints from a lake site in Southern Australia dated to about 60 ka, and possibly as much as 70 ka (I believe investigations are continuing on this site. And at this time of the day, I can’t be bothered to look up the site’s name. It has been mentioned here before, several times. Until it’s proved wrong (which would be quite major news), it’s an important early anchor point in the distribution history of humans.
Yes, I think archaeology has pinned the first “human” (by skeletal morphology, ichnology, artefacts and likely behaviours) exit from Africa to Australia (and intervening locations) considerably before “50-60 ka BP”, and that has been the conventional opinion for the thick end of a decade.
The last time I saw an “integrated” “human” dispersal history, it had multiple “Big Exits” some time before 2Ma, probably around 100 ka, and probably around 60 ka (all BP).
Nice to see the molecular people are keeping up to date.
I checked : Wiki gives dates of “1.85 to 1.77 million years old” for Dimanisi, reported from 2007 to 2013 ; these excavations have been going since 1936, with “modern” work from around 1982 to today (dating coming in through the last several decades) ; the usual arguments between lumpers and splitters over which hominid/ hominin group to ascribe the skeletal remains to.
(While I’m digging for the Australian lake trail … I forgot Homo luzonensis in the Philippines.) Ah, it’s Lake Mungo I’m thinking of (probably – this refers to skeletal remains, but my memory is clear on the site being a trackway ichnofossil, not a body fossil. Whatever.) The body fossils have a chequered history of dating, between “24,600 ± 2,400 and […] 62,000 ± 6,000 years.” Dating is difficult.
It’s also late here. It remains nice to see the molecular data coming into line with the other data.
“The conventional wisdom is that all of these subspecies and species went extinct until “modern” Homo sapiens made its Big Exit into Eurasia about 50,000-60,000 years ago….” I don’t think so: Neanderthal bred with sapiens ~45,000 years ago, and floresiensis may have lived well beyond Toba’s eruption and subsequent chill. Luzonensis was said to be 67-50,000 years, possibly after the Toba chill/die.
“That lineage split into the sister subspecies Neanderthals and Denisovans (I consider them subspecies of Homo sapiens)”
They being subspecies helps to explain how Homo sapiens could mate successfully with them.
Hi Jerry.
You said that Homo erectus, which might have left Africa about 1.75 mya and then spread to eastern Asia by 1.5 mya, died off for reasons unknown.
Did they die off or did they leave descendants such as heidelbergensis? If they left descandants, then that’s not the same as dying off.
David Lillis
No, they did not leave descendants.