Tag Archives: Homo sapiens

The Inheritors

42.4-40.2 thousand years ago

“The Inheritors” is a novel by William Golding about the encounter between Neanderthals and modern humans. Like another Golding novel, “Lord of the Flies,” it is written with a mid-twentieth century awareness that advanced societies don’t leave behind the potential for cruelty. The novel isn’t all that scientifically accurate, though: Golding’s early humans have bows and arrows, for example.

Neanderthal 1, the first Neanderthal fossil recognized as probably belonging to another species, was discovered in the Neander Thal (=Neander Valley) in 1856. He is close in time to the last Neanderthals: the most recent review of the evidence finds that Neanderthals disappear as a distinct group around 40 thousand years ago. They were almost certainly outcompeted by Homo sapiens who had arrived in Europe earlier. There was probably some kind of coexistence between Neanderthals and H. sapiens over many thousands of years. Regional cultures from this period, like the Châtelperronian, may represent Neanderthals copying the Aurignacian culture of incoming H. sapiens. The final blow may have come between 39 and 38 thousand years ago, when a “Heinrich event” sent cascades of icebergs into the North Atlantic, drastically chilling Europe. Homo sapiens recovered from the cold spell; Neanderthals did not.

Just last year we have learned that one of the earliest modern human fossils from Europe, Oase 1, from Romania (40 kya), has substantially more Neanderthal DNA, 6-9%, than living Europeans. Furthermore, this DNA comes in the form of long stretches of chromosomes, rather than little bits broken up by millennia of recombination, showing that his Neanderthal ancestors go back just a few generations, maybe to some great-great-great-grandparents. We’ve also learned from isotopic evidence that Oase 1 got the proteins in his diet from a broad array of sources, including freshwater fish.

And here is some of the art produced by Homo sapiens 40 thousand years ago. lionman The lion man from Hohlenstein-Fels

And the earliest known cave paintings, from Sulawesi, Indonesia. indonesiancaveart

Species

We have been treating Neanderthals here as a species, Homo neanderthalensis, distinct from our own species, Homo sapiens. Some researchers elect to call Neanderthals a subspecies, Homo sapiens neanderthalensis, and classify modern humans as another subspecies, Homo sapiens sapiens.

The line between subspecies and species is not clear cut, nor – given the way evolution works – should we expect it to be. Recent work on ancient DNA recovered from fossils has shown just how complicated the subject is. The spectacular finding of the last few years is that modern humans are hybrids, getting most of their ancestry from a single founding population (we can call them Homo sapiens), but incorporating limited ancestry from close relatives. Thus human beings outside Africa have 1-4% Neanderthal DNA. So it looks as if early in the course of expansion(s) out of Africa, there was a limited amount of interbreeding with Neanderthals.* And not just with Neanderthals. Populations in Melanesia get an additional 4-6% of their DNA from a widespread East/Southeast Asian population known as Denisovans, while some African groups may have ancestry from non-sapiens populations in Africa. (The fossil record for Denisovans is a lot sparser than for Neanderthals, and it’s even sparser for African non-sapiens.)

This isn’t reason enough to put Neanderthals and sapiens in a single species: plenty of species occasionally hybridize with related species. And in fact the DNA evidence implies that sapiens and Neanderthals were moving toward being reproductively isolated. Specifically, we find that a lot of Neanderthal genes related to testis development and male fertility are underrepresented (i.e. at a lot less than 1-4% frequency) in modern humans. The likely explanation is that those genes didn’t work well against a H. sapiens genetic background. In other words, if you were mixed sapiens/Neanderthal man, you probably had fertility problems, albeit not to the point of complete sterility. On the other hand, other Neanderthal genes  – especially genes related to immune function – were useful to modern humans moving into Neanderthal territory and are found at high frequency in Europeans today.

There is an extensive older literature in physical anthropology on “race crossing.” Researchers were concerned with whether people with mixed racial ancestry might have reduced fitness as a result of combining incompatible genes. This literature is reviewed at book length here. The overwhelming evidence is that “race crossing” has no harmful biological consequences (in contrast to close inbreeding, which is a bad idea: check out this post on the Habsburgs.)

Some new data put this in perspective. My colleagues Alan Rogers, Chad Huff and Ryan Bohlender have just shown that the population ancestral to Neanderthals and  Denisovans separated from the ancestors of Homo sapiens somewhat more than 750,000 years ago, passing through a narrow population bottleneck, probably in an early Out Of Africa episode (before the later modern human Out Of Africa episode). This population then split somewhat less than 750,000 years ago into Neanderthal and Denisovan branches. So Homo sapiens and Homo neanderthalensis evolved separately for the better part of a million years, and were some way on the path to reproductive isolation. By contrast, different populations (“races”) within Homo sapiens have been evolving separately for 100,000 years or less outside Africa, and perhaps 250,000 years within Africa. This has been enough time to evolve major differences in traits like skin color and hair form, but apparently not to create appreciable biological barriers to interbreeding.

And here’s a link covering some recent research suggesting that across a wide range of organisms it takes a surprisingly clock-like average of two million years separation to split one species into two.

* Hence, my Neanderthal name is Carg, and my website is cargshome.

Australian megafauna and the Sixth Great Extinction

The Earth has been through a number of mass extinctions. On Logarithmic History, we covered the five greatest mass extinctions during the month of March, and into early April.

For all these extinctions, the most likely cause is some kind physical catastrophe: either drastic changes in the chemistry of oceans and atmosphere, or extraterrestrial events, like asteroid strikes or, just possibly, gamma ray bursts.

Competition with other organisms is another major cause of extinctions. Usually this is just part of a steady background level of extinction. But occasionally competition – and extinction rates – increase dramatically, as during the Great American Interchange five million years ago, when South American animals were swamped by North American invaders with the establishment of the Isthmus of Panama.

Earth now may be on the edge of another episode of mass extinction, a Sixth Extinction. (although rates of species extinction don’t match those of earlier ME’s). This time the cause is very different: a single species, Homo sapiens, is playing an overwhelming role. Although the pace of extinction has accelerated over the last few centuries, you can make a case that the sixth extinction began a long time ago, with the expansion of modern humans out of Africa. Australia in particular sees the disappearance of a unique fauna that evolved over more than a hundred millions of years of isolation. This fauna included monster wombats, giant kangaroos, huge flightless birds, and a marsupial version of a lion.

australiamegafauna
All of these – all land mammals, reptiles, and birds with mass of more than 200 pounds – seem to have gone extinct by around 45,000 years ago, after humans crossed the sea to settle the island continent. (Sea levels were lower then, and Australia was connected with New Guinea, but still isolated from mainland Asia.) This is not settled science. Some researchers think climate change was to blame. But I think the evidence in the case of this and later mass extinctions is strongly in favor of humans as the major agent of extinction. This is one more reason to treat the advent of our species as one of the major evolutionary transitions, comparable to the evolution of complex cells, or multi-cellular life.

Two roads diverged

53-50 thousand years ago.

The broad outlines of the spread of Homo sapiens have been established for several decades now: origins in Africa, expansion out of Africa at least 50-45 thousand years ago. But we’re still arguing about the details. Just this year it’s been reported that modern humans reached Sumatra 73-63 thousand years ago, and that stone tools in Australia date back 60 thousand years ago.

Also, based on recent recalibrations of DNA mutation rates, it looks like the African/non-African split might have happened more like 100 thousand years ago than 50 thousand years ago. So the ancestors of non-African (or non-sub-Saharan-African) H. sapiens might have occupied a homeland somewhere north of the Sahara between 100 and 50 thousand years ago, before spreading through Eurasia. North Africa is one possibility. The Near East, maybe the Arabian peninsula, is another possibility. The (or “a”) homeland might be (gated, sorry) underwater, under the Persian Gulf (sea levels were lower then). Both possibilities have some archeological support. There might have been multiple homelands, and multiple expansions – south through Arabia and along the shores of the Indian Ocean, and north through the Levant and into Europe.

A recent (2015) redating of archeological finds suggests that the Levant-to-Europe corridor was part of the story. A modern stone tool technology, coming from Ksar Akil, just outside Beirut, Lebanon, dates to about 50,000 years ago, a little before much the same technology appears in Europe, in the form of the Upper Paleolithic.

And many other “details” remain to be resolved: What did interbreeding with non-sapiens mean for the evolution of H. sapiens? And just what advantage(s) did H. sapiens have that allowed him (us!) to replace other species? Stay tuned for more on Logarithmic History

Toba? or the sperm whale effect?

74 thousand years ago, a big chunk of the island of Sumatra blew up. It was the biggest volcanic explosion in the past two million years, expelling 2800 times as much debris as the Mount Saint Helens eruption in Washington State in 1980. Ash from the super-eruption is found all the way from Lake Malawi to the South China Sea. The resulting Toba caldera measures about 20 by 60 miles.

toba

The Toba eruption coincides with a shift back to glacial conditions, and it may be that there’s a connection, that Earth went through a long volcanic winter after the eruption, which shifted climate to a colder equilibrium.

Did Toba have an effect on human evolution? Somewhere between 100 and 50 thousand years ago, human populations went through a bottleneck: modern humans are descended from just 1,000 to 10,000 breeding pairs from that period. It’s been argued that Toba wiped out the majority of Homo sapiens around at the time, leaving only a small group of survivors.

But the evidence that Toba is responsible for the bottleneck is equivocal. In some places humans seem to have passed through the period of the eruption without major disruptions. Also, there’s a point that gets missed in a lot of popular reporting: just because a species went through a bottleneck doesn’t necessarily mean that the population of the whole species ever shrank to that size. In the case of Homo sapiens it could be that the total population was always many times larger than 1,000-10,000. It’s just that the other tens or hundreds of thousands got replaced. In other words, we may not be looking at an external catastrophe wiping out most of humanity, and a few groups of survivors recovering. Instead, we may be looking at a small population of our eventual ancestors expanding and outcompeting other populations, so that it was our ancestors, not a volcano, who made sure that most human beings alive 74,000 years ago didn’t leave descendants.

This may reflect something special about human evolution: human beings typically belong to tribes and ethnic groups defined by distinctive cultures, and cultural boundaries (including language boundaries) often act as barriers to interbreeding. Several authors have suggested that this may make human beings unusually susceptible to population replacement via “cultural group selection,” and that this might account for humans having unusually low effective population size, as genes “hitchhike” along with expanding cultures. Interestingly, sperm whales, which live in populations defined by different song dialects (and other cultural differences) may show the same genetic pattern.

In May 2015, the Toba volcano grew more active than usual, producing large emissions of steam and foul gases. Locals were reported to be concerned.

Blombos

75 thousand years ago

The period between the time Homo sapiens leaves Africa 120 thousand years ago, and the time when H. sapiens spreads far and wide through Eurasia, replacing Neanderthals and others, 45 thousand years ago, sees episodes of increasing cultural complexity in Africa. One of these occurs at Blombos cave, at the southern tip of South Africa. There, over tens of thousands of years, people make a cultural great leap forward.
blombos2
75 thousand years ago, we find that they produce finely crafted stone blades that are part of multi-part composite tools. They make shell ornaments.

blombos

And they etch ocher (a red stone useful as a dye, but not for tools).

Oddly though, this tradition doesn’t last. It’s over by 60 thousand years ago. It may be that people left as climate deteriorated. But Blombos cave is a reminder that cultural progress is not always a permanent thing. It looks like an early instance of Rise and Fall: a culture rises to new heights, and then falls back.

African Exodus

123-117 thousand years ago

Our genus, Homo, left Africa by 1.8 million years ago. There may have been another Out-Of Africa migration around 200,000 years ago that introduced early Homo sapiens mitochondrial DNA into the Neanderthal gene pool. This early migration didn’t lead to major evolutionary change, however. Other Neanderthal genes (i.e. nuclear rather than mitochondrial DNA) show no evidence of it.

More consequential was another Out Of Africa migration by Homo sapiens around 120,000 years ago. This skull dates back to then.
skhul5
Skhul 5 was found back in 1935, in Israel, on the slopes of Mount Carmel, buried together with a boar mandible. It looked for a long time like the skull came late in the day, and might represent a transition from Neanderthals to Homo sapiens. But now that date has been moved back, thanks to the development of new dating techniques (thermoluminescence, electron spin resonance) that finally broke the 40,000 year limit for Carbon 14. Skhul 5 now looks like a representative of an early movement of Homo sapiens out of Africa. But the skull also has some Neandethal-like features (check out the “Neanderthal bun” at the back of the skull) and could have hybrid ancestry.

Until recently it looked like this and other very early Homo sapiens outside Africa were a side branch that left no descendants – think Leif Erickson, not Columbus – with the real move coming later. But recently there has been a recalibration of DNA mutation rates that suggests that the split between African and non-African branches of H. sapiens happened closer to 100,000 years ago than 50,000 years ago. And there have been discoveries of stone tools with African affinities in the Arabian Peninsula, in the United Arab Emirates (Jebel Faya, 125 kya) and Oman (106 kya). It may be that when Homo sapiens left Africa 125,000 years ago (perhaps across the Red Sea to Arabia, rather than across the Sinai), they spent a long time isolated in a corner of Southwest Asia before much later expanding more widely.