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Saturday, 8 August 2020

Out of Africa 2, Evolution of Homo Sapiens

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Are Neanderthals and Anatomically Modern Humans the same species?

Before talking about the origin of Homo Sapiens, we need to consider (just concentrating on the major groups) whether to include include Neanderthals and their close cousins, the Denisovans.

Neanderthal woman, Bacon Cph 

Neanderthals evolved in Europe and ranged across southern Europe, the Middle East and Asia as far as the mountains at the western edge of Mongolia. They survived until about 40,000 YA. 

The closely related Denisovans (from ‘the cave of Dennis’ in Siberia) were a little more elusive. They ranged mainly across Asia, surviving in New Guinea perhaps as late as 11,500 YA.

 In Western Africa, there were pockets of archaic humans that survived through till at least 13,000 years ago.  

Modern Human DNA across Eurasia and New Guinea contains DNA from Neanderthals and Denisovans, indicating ‘AMHs’ (Anatomically Modern Humans) bred with them. Modern African DNA indicates interbreeding with an, as yet unidentified, archaic human.

They have found bone fragments of hybrids: an AMH/ Neanderthal hybrid (in Italy) and a Denisovan/ Neanderthal hybrid (in the above mentioned Siberian cave), both with Neanderthal mothers.  There is a more complete specimen from Romania of a 15-16 year old male with mixed modern and archaic anatomical features dated as recently as 30,000 YA ago.

If AMHs, Neanderthals and Denisovans interbred (and the consensus is that they did) perhaps they  should be part of the same species, they should all be called 'Homo Sapiens', Homo Sapiens Neanderthalis. 

I have considerable sympathy for this view, so I have stuck to the somewhat clumsy term ‘Anatomically modern Humans’ (‘AMHs’) for the African version (rather than Homo Sapiens Sapiens). 

I have to emphasise however, that this is not the consensus. Most see them as separate species and the situation with inter-breeding is not so simple.  

For convenience I will only concentrate on Neanderthals, though Denisovans genes were more successful than the Neanderthal ones. 

Some gene studies have suggested successful interbreeding with Neanderthals might have been uncommon. If this were true, and even if it was just from lack of physical attraction, the further we get from successful interbreeding, the closer we get to them being separate species. 

The Y chromosome and mitochondrial DNA

There is no Neanderthal DNA in modern human mitochondria and not a lot in Y chromosomes. This has led some anthropologists to variously suggest that Neanderthal/AMH hybrids were infertile, aborted, or sickly. Again, if this is true, the further away from viable, fertile offspring we get, the closer we are to species separation (‘speciation’).

The mitochondrial DNA and Y chromosome stories point in different directions.

Mitochondria are small factories in cells that produce energy and protect the cell from oxidative enzymes. They have their own DNA, which mutates more rapidly than nuclear DNA, and all mitochondria and mitochondrial DNA comes from the mother.

Neanderthal m-DNA is very distinctive. If there is  none in modern humans, it suggests that a hybrid with a Neanderthal mother is less viable or might be infertile, and not the cause of our Neanderthal genes.

Moving back to the DNA found in cell nuclei : women have two X chromosomes, men have an X and a Y.  It is men , through their sperm, that determine the sex of the child.

While there is a mechanism for a pair of chromosomes to sometimes swap matching parts of themselves, sexual reproduction is the main way advanced forms of life have of mixing genes. (Swapping genes can happen between X and Y but not in the sex-linked portion).

So if there is less Neanderthal DNA in the Y chromosome, maybe it is the boys with a Neanderthal father that could have been infertile or have low survival.  

Other reasons for these findings

All these possibilities are not necessarily mutually exclusive and there are also are other possible factors that might have contributed to these genetic findings.

The above may be due to sampling errors. Gene technology is very new (initial draft for Neanderthal genome May 2010). We have few Neanderthals to test and even fewer hybrids and it is the hybrids which will contain the answer. 

Now we can identify hybrids even from bone fragments, and investigate them, perhaps we can see what really happened.

As mentioned, Neanderthals had fewer numbers than European AMHs (Cro-Magnons). They many have left a smaller genetic footprint from simple dilution.

And yet, there is a darker possibility for our poor Neanderthals and unfortunately it seems to be at least part of the story.  

Neanderthals were a physically robust group and physically well adapted to hostile, changeable climate in the Glacial period. Still, they mostly only lived till 35 whereas Cro-Magnon adults often lived till 50. 

Several studies of Eurasian fossils suggest initial levels of Neanderthal DNA were about 3 times what is seen today and a lot of the surviving Neanderthal DNA is 'non-coding' (so called 'junk' DNA). 

It seems natural selection was particularly brutal towards Neanderthal DNA, and it acted over a relatively short period of time.

Some paleo-geneticists suggest that Neanderthal populations were more in-bred, with harmful or weak genes. Whether this was true or not, there is no doubt Neanderthals were seriously outmatched by AMHs.

They had simpler tools with little adaptation over time and simpler social organisation and clothes whereas AMHs literally specialised in innovation. AMHs made throwing spears 279,000 YA and 130,000 Y.A.. Even before leaving Africa they developed art, pigments, glue, decorations, beads, and multiple tools and traded (obsidian) over 300 KM distances while Neanderthal trade networks were much shorter

By 60-70,000 YA. AMHs had developed hafted tools and bone and stone arrowheads ( 60-70,000 YA).

When AMHs finally left Africa, they were in small numbers (see below) and yet the Neanderthal phenotype lost really badly and fairly quickly, only overlapping Cro-Magnon man by 5,400-12,000 years. 

I won't document all the theories for extinction of Neanderthals, but some suspicion has to fall on the arrival of AMHs, whether they simply 'out competed them' and brought new diseases. There is less support for early theories that they hunted them.  

Chrono-species

Another factor that might explain the apparent interbreeding of two 'species' would be the concept of 'chrono-species'. All (or most) of the species of Homo are ‘chrono-species’ (as discussed in last month's blog). 

Take the Chihuahua and the Great Dane. They are sufficiently different that they cannot breed (even if they wanted to). They would be separate species, except that there is a chain of dogs in the middle that allow them to share the same gene pool.

In ‘chrono-species’ there is increasing separation over time and the intermediate links die out, leaving two or more  separate species. When this separation occurs is difficult to say, and if we go back in time to when the species have not fully separated, (for example to advanced versions of H. Erectus and early H. Sapiens), we might very well get interbreeding, though we would consider them different species from the point of view of a latter point in time.

(On the same vein and while we don’t think this is what happened, if AMHs and Neanderthals both bred with an archaic link and not each other, this would allow an exchange of genes while they are separate species).

Back to the question, where did H. Sapiens evolve?

H. Erectus, coming from Africa, colonised a large part of Eurasia starting about 2 million years ago, though in nothing like the numbers of AMHs. 

Where did AMHs evolve?

The earliest specimen of AMHs were found in North Africa  (Jebel Irhoud, Morocco about 300,000 years ago).

At first there was a mix of ‘modern’ and ‘archaic’ features. The modern skull features are a straight high forehead with a box-like head , smaller teeth and a prominent chin. The more ‘archaic features’ involve a thicker skull, the low, receding forehead, a somewhat oval skull, distinct brow-ridges and prognathism.

Over time, AMHs approached something more modern ( e.g. ‘Herto man’, Ethiopia 160,000 years ago progressing to another specimen from Tanzania 120,000 years ago).

They also developed a new (middle) Stone Age tool technology called ‘Aterian’.

AMHs spread to Israel/ Palestine and the Arabian peninsula fairly early. 

These regions had much less desert at the time and shared a number of animals with Northern Africa, the habitats being virtually connected.

The first specimen outside of Africa was a jaw bone fragment found near Mount Carmel in Israel,  dating back to about 190,000 YA. The characteristic Aterian tools have been found in Israel and the Arabian Peninsula also dating to about this time.

There is equivocal evidence (damaged fragments) of a surprisingly advanced form of AMHs in Greece 200,000 YA. 

Characteristic teeth and tools have also been found in China dated to 100,000 YA and modelling suggests AMH genes entering the Neanderthal gene pool around 100,000 YA.

While AMHs probably started in Africa, they would continue to evolve (and swap advantageous genetic adaptations) over the whole of whatever territory they had spread to.  

If it is confirmed that a truly advanced form made it to Greece so long ago, this might challenge the current narrative, but for the moment it seems that AMH first evolved in Northern Africa and spread out (with a mix of archaic and modern features) at least to the adjacent parts of the Middle East.

And then disaster struck (more on this in a minute).

3 models of H. Sapiens evolution

There are three main theories of how Homo Sapiens evolved from H. Erectus.

The multiregional hypothesis, as first described, suggests that so called ‘major racial groups’ arose because H. Sapiens emerged in multiple sites.

 Modern Australasians evolved from an Australasian H. Erectus, and so on. While there are dotted lines at each level allowing genetic mixing, this theory could be used to infer that the separation of modern man into so called ‘racial groups’ is much more ancient and fundamental than it actually is.

The ‘out of Africa’ theory suggests H. Sapiens evolved in Africa and then spread to Eurasia and might have been involved in pushing the other ‘archaic humans’ into extinction. 

Neither of these theories fully match the facts.

The Assimilation model looks a little closer to me: AMHs evolved in Northern Africa and they partly assimilated the archaic humans they came into contact with.

A time of Disaster   

Around 200,000 YA. was a good time for AMHs.

The world had been cooling for tens of millions of years but for that moment was going through a wetter, warmer phase and the Sahara was green. .

Still, from around 350,000 YA (the period when the AMHs, Neanderthals and Denisovans were evolving) the climate was becoming increasingly unstable. It was beginning to alternate between warmer , wetter periods and cold, very dry, periods. 

It would have been a challenging time, and maybe helped spur faster evolution.

Then, around 115,000 YA, the climate turned murderous.

A Glacial Period began in Northern Eurasia (and North America) resulting in shrinking habitats and it lasted through till 11,700 YA.

It was worse than that, the cold diverted rains away from Northern Africa and the Middle East triggering mega-droughts that went on and on and on, with massive desert expansion .

But something worse, far worse, was just about to happen, a great cataclysm.

Around 75,000 YA  Mount Toba in Sumatra  erupted. 

The eruption was 12 times more explosive than the largest volcanic eruption in recent history (the 1815 eruption of Mount Tambora in Indonesia, which caused the 1816 ‘Year without Summer’ in Europe).

Some say it had little impact on the environment, others say it caused a ‘nuclear winter’ (if one can still use a somewhat debated concept). Whether this was the cause or not, this time saw a massive depopulation event, involving several mammalian species.

The groups of AMHs outside of Africa are thought to have died out, any in Europe were later replaced by Neanderthals and back in Africa, they were pushed to the brink of extinction.

Genetic models point to a serious ‘population bottle neck’ in those AMHs that survived in Africa (a sudden reduction in genetic diversity).

It was almost the end for our Anatomically Modern Humans.

Today's humans are said to be descended from a very small population,  between 1,000 and 10,000 breeding pairs (70,000 YA) and the common female ancestor of all living humans on earth (mitochondrial Eve) may have lived as recently as 150,000 YA.

Recent ‘out of Africa’

We now believe that a very small group of AMHs left Africa (this time successfully) 70,000–50,000 years ago and mainly followed the Southern Route to the Arabian Peninsula (sea levels were lower then but water transport was also available to them). 

They continued relatively rapidly along the coast of Asia to reach Australia around 65,000–50,000 years ago.

An early offshoot followed a northern route via the Levant to West Asia, finally reaching Europe around 48,000 YA..

Behavioural Modernity

There are subtle physical differences between Cro-Magnon and the humans of today, showing we have been continuing to evolve, even over recent times, but the intellectual difference between Neanderthal and Cro-Magnon Man (40,000- 10,000 YA) was stunning.

Cro-Magnon, Charles Robert Knight,1920
While Neanderthal was still an advanced form of homo, Cro-Magnon had a rich culture with sophisticated stone working, advanced symbolism and cave paintings that stood in stark contrast to our poorer cousins.

It is described as 'behavioural modernity'. Some saw it
as an adaptation to the harsh environment in Europe at the  time but, as mentioned, it was already present in 
considerable measure and gaining momentum before AMHs left Africa. 
It is an area in which (we) AMHs really excelled, 
especially innovation, and it led to our success.

It goes without saying that Homo continues to evolve slowly and subtly, with improved adaptation to the environment in which we find ourselves and sharing these genetic adaptations across a gene pool now in the billions. 

Lately this has been greatly overshadowed by social evolution (technological and social revolutions), where we are literally creating these new environments. The amount of change has been extraordinary and the pace is getting faster all the time, even in living memory.

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