Thursday 9 July 2020

Out of Africa 1, the Evolution of Man

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Primates evolved 55 million years ago (MYA) ago in a world very different to our own.
Crab-eating_Macaques, Bernard Dupont, 

10 million years before, there had been a massive asteroid strike (or strikes) causing a 'mass extinction event' including most non avian dinosaurs, initiating a golden age for mammals.
The main land masses were closer than today. Eurasia and America were just separating; Australia and Antarctica were much closer and India was still a large island south of Eurasia.
It was very hot and steamy, in no small part due to greenhouse gases (later sequestrated into our fossil fuels and lying frozen in Siberian peat bogs). Equatorial forests stretched all the way to Antarctica.
It was an ideal world for the evolution and spread of a teeming population of tree dwellers and small mammals.

But it was also, for several reasons,  a climate maximum, heralding an inexorable cooling over many millions of years leading up to the Last Glacial Maximum about 19,000 BC.
Over this long period, tropical forests eventually shrank to the equatorial regions, being replaced by deciduous forests, grass lands and deserts. Permanent ice eventually formed at the poles. (Non human) primates eventually retreated from Europe, leaving only the Barbary apes of Gibraltar.
Current Primate range Napier and Napier, Wiki

This change was slow and 25 MYA there was still plenty of space for the Great Apes to evolve and flourish in the equatorial forests, through till about 15 MYA when the shrinkage of their habitat really began to 'bite'.

From our tree-dwelling forebears mankind inherited  forward-looking eyes, acute binocular vision (more often seen in predators) and grasping hands .
While there are many interesting suggestions, there is no fully accepted explanation of why our forebears moved to an erect posture and a bipedal (striding) gait, as opposed to what other apes and monkeys now do when they walk. Maybe it was a combination of things.
There is, however, a prelude in how apes use their upper limbs for grasping, manipulating (feeding and grooming) and their rear limbs for walking along branches. Apes also they tend to sit erect when feeding or resting. The potential for an erect posture and bipedal gait  was already there and that was the way 'we' 'decided' to go, and it worked .
Most Apes and monkeys remained in the shrinking jungles, but some monkeys (e.g. baboons), and some apes (e.g. chimps and Australopithecus) moved into the savanna (mixed grass/woodland) that was spreading out over East and Northern Africa.
To use a very archaic term, the Genus Australopithecus could be described as a ‘missing link’ between Apes and Homo (having features of both). They evolved around 4.2 million years ago, becoming widespread, eventually dying out about 1.9 million years ago.
They were the first of our ancestors to walk erect on two legs, sleeping in the security of trees at night. They had brains roughly 35% of the size of a modern human brain. Most were slender and covered in fur. The larger of the Genus reached 1.4 meters, many were smaller and females were distinctly smaller than males.

The name 'Australopithecus' comes from a combination of Greek and Latin, meaning ‘Southern Ape’ (from a full specimen discovered in South Africa).
They lived on fruit, vegetables, and tubers, and perhaps easy-to-catch animals such as small lizards.
The ‘Genus’ Australopithecus is defined as a group of ape species and subspecies that had features between apes and ‘homo’. In other words they walked erect.
We have no way to be sure if they are all as closely related as we think, we have lumped them together on similarities, often on the basis of fragmentary evidence. Some anthropologists variously propose lumping them with homo, splitting off some of them into other existing genera or using additional suggested genera to subdivide them. see here
This problem of classifying their finds is one of the great challenges for archaeologists and anthropologists who use morphology, habitat and behaviour as a guide to species, subspecies and genera (more on this later).
Cutting ties 
Genus Homo diverted from the Genus Pan around 5 million years ago, as measured by differences in DNA. Genus Pan is our closest relatives amongst the apes, chimpanzees being their only surviving representatives. 
In Genus Homo two chromosome pairs fused to reduce our chromosomes to 23 pairs instead of the 24 pairs in other apes, decisively separating Homo from all other surviving apes.
We currently need living, dividing, cells to count chromosomes, so we don’t know when this reduction happened, but it is most likely early. We also don’t know if Australopithecus had this too, but it is definitely possible.

Genera, Species and Subspecies
The concept of a species seems simple enough: (the largest grouping of organisms that can crossbreed with one another and produce fertile offspring).  A ‘genus’ is a group of related species with a common ancestor, and subspecies are groups within a species that look different, sometimes significantly so, but can still readily crossbreed if given the opportunity.
'Polytypic' can refer to a Genus (with lots of species) but more often refers to a species with lots of subspecies. The main species of homo over time were polytypic.

‘Breeds’ of domesticated species (like the multiple types of dogs, horses etc) are a special example of ‘subspecies’. If dogs were somehow long extinct and only a few partial remains were available, an archaeologist would likely think he had discovered several different species. In fact domesticated dogs, some wild dogs, and various wolves are all (currently) recognised as one species!

Early man cannot compete with the sort of diversity of Canis Lupus Familiaris (domestic dogs) has/have. The evidence is still fragmentary, especially in the distant past, but over time we have begun to believe that interbreeding of different-looking contemporary forms of Homo was far more common than we ever dreamt could be possible, and this explains some of the combinations we have found.
It's hard to be absolutely sure, but much of what we assumed were different contemporary species were subspecies. 
Part of this new understanding followed advances in understanding of how evolution actually works, and part of  it followed the realisation that modern man interbred with Neanderthals and Denisovans as soon as he reached Eurasia from Africa (more on this later).

Evolution and subspecies.
Evolution occurs when there is variation between individuals (within a species) and some of these variations have an advantage for survival and reproduction, either dependent on a certain habitat or an over all advantage.
Sub-species occur in nature when groups of the same species become separated over time or place and they then adapt to different environments or simply experience genetic drift. 

With several subspecies of early man (capable of interbreeding). Each had particular mutations that might be an advantage 'over all' or in certain habitats. Interbreeding, ensured that any differences that might prove advantageous could be combined and not lost forever over time.
Specific combinations may work well together. Such a wider variation allows penetration into niche habitats and faster adaptation to habitat change so was often a survival advantage for the species.

This interbreeding and evolution within subspecies leads to the concept of 'chronospecies'. A chronospecies is a species which slowly changes over time until it cannot be considered to be the same species as the originating genetic line (had they existed at the same point in time). 
Chrono-species Ian Alexander, Wiki

It might be just genetic drift and/or different environments causing such an accumulation of differences that eventually interbreeding becomes unlikely (e.g. loss of sexual attraction, mating rituals or physical differences).

While natural selection can favour  interbreeding to foster a larger gene pool with greater diversity, it can also favour separation of subspecies into separate species ('speciation') .
This happens when the 'in-between' or older varieties become at such a disadvantage relative to the new variety that the benefits of the larger gene pool (to the species as a whole) is outweighed by the cost.
Such a thing occurs more rapidly when the habitat of the subspecies is very different and the adaptation needed for each niche habitat is different.

In the development of a chronospecies, each change is small and interbreeding occurs along the 'chain'. The point at which we would say a new species has 'arisen' is arbitrary. The evolution of man followed just such a pattern.

While there is considerable uncertainty and debate, the minimalist view (currently favoured) leaves Homo Habilis, Homo erectus and Homo sapiens (and their respective contemporaries) as the only three species of Homo.  
Encyclopaedia Britannica  see here
Gone are the multiple ‘species’ that were proclaimed every time an old time archaeologist found remains that varied from what had been found before.

We need to be cautious not to see evolution as always a march from primitive to more advanced and complex, though this can happen.
Natural selection doesn't care about human value judgements, it only cares about adaptation (survival) and reproduction. It just so happens that many of the adaptations leading to modern man had a huge survival advantage. 

Homo Habilis
Homo habilis was one of the earliest species of ‘man’ (homo), 2.4 to 1.4 MYA (overlapping Australopithecus by half a million years).
The arrival of Habilis marks the official start of Genus Homo.
‘Habilis’ is Latin, the same root as ‘able’ and has a number of related meanings like ‘skilful’. ‘Homo Habilis’ has been translated as ‘handy man’, tool maker.
Female H. Habilis, √Člisabeth Dayn√®s 
We initially used evidence of fashioning of tools as the  defining feature of Genus Homo, but advanced types of Australopithecus have been shown to fashion primitive tools maybe even a million years before Homo arose.
We don’t know for sure if Homo Habilis is a direct ancestor of modern man as we first believed, and we don't know 'his' relationship with a number of other early forms of Homo (like H. rudolfensis, another contender for the title of being our ancestor. We don't know whether they were separate species or subspecies).
The oldest Homo remains, LD 350-1, (as yet unclassified into a sub species) is from Ethiopia 2.8–2.75 million years ago (mya) confirming that 'homo' evolved in the African savanna.

Homo spread to Asia about 2 million years ago. There were manufactured tools from 2.1 MYA found in Shangchen, China and specimens found in the nation of Georgia 1.85–1.77 MYA. These were thought to be from Homo erectus but the Georgian specimens and brain size are decidedly primitive, so it might have been a variety of Homo Habilis or maybe something in between.

Homo Erectus
The award for the first species of man that spread beyond Africa usually goes to Homo Erectus.
H. Erectus lived from between 2 MYA to as recent as 110,000 years ago, overlapping H. Habilis by almost half a million years and Homo Sapiens by about 200,000 years. Being a Chronospecies the later versions of H.Erectus would be more advanced.
H. Erectus dispersal, Sheila Mishra Here
It is the longest surviving of all human species, being present nine times as long as our own species, Homo sapiens.

H. Erectus was highly successful, and mobile, and spread widely over Africa and Eurasia
It was H. Erectus that first began to hunt medium-large animals in groups (such as bovines or elephants), moving to a diet richer in meat. Erectus would have had some tribes that routinely followed migrating herds as well as moving into other niche habitats.

Erectus tamed fire a million years ago , and maybe earlier, and he made water craft probably for fishing, but was able to colonise at least a few islands (eg Indonesia)

With longer legs and relatively shorter arms (no longer climbing trees) loss of hair, and adaptation in sweat glands, he was able to run longer distances in the heat . H. Erectus became bigger, faster and smarter (with an average brain capacity at least a third bigger than H. Habilis).

There is a conflict in the best pelvis for running on two feet and the ability to deliver babies with large heads.
In 'Modern man ' children are dependant on their parents for longer (with the continued move to more monogamous bonding in parents) allowing considerable brain growth after birth.
Larger brains require a lot of energy and are best supported in a hunter-gatherer society by cooking food and using more meat in the diet, requiring co-operative hunting of large animals, mostly by males working together.

H. Erectus is a chronospecies with a very long history and a very wide range. He evolved over time and there were many subspecies. The larger subspecies had body proportions similar to modern man, with distinctly larger brains (up to 60% of H. Sapiens). This is in sharp contrast to the smaller and earlier subspecies (or species) of Erectus.

The main African versions is often called ‘H. ergaster’.
Homo heidelbergensis (700,000 to 300,000 years ago in Africa and Europe) is the subspecies (or maybe chronospecies) closest to H. Sapiens.

The oldest H. Erectus remains are found in Kenya. The majority view is that H. Erectus also evolved in Africa and spread over Euro-Asia via the Middle East then on to Asia (including China and Indonesia) about 2 MYA. As said, he also made it to islands and must have made primitive water craft. There is some debate when H. Erectus arrived in Europe, whether it was soon after or somewhat delayed because ‘he’ preferred a warmer climate.

Next blog is the conclusion of  'out of Africa'

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