Out of Africa 1, the Evolution of Man

Primatesevolved 55 million years ago (MYA) ago in a world very different to our own.

Crab-eating_Macaques, Bernard Dupont, Wiki

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.

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) with equatorial forests stretching 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 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 impinge on the species.

From our tree-dwelling forebears mankind inherited  forward-looking eyes, acute binocular vision (more often seen in predators) and grasping hands .

There is no clear explanation of why our forbears moved to an erect posture and a bipedal (striding) gait when they left the trees, despite some very interesting suggestions.
There is a prelude of this in how apes use their upper limbs for grasping, manipulating (feeding and grooming) and their rear limbs for walking along branches. Also, they tend to sit erect when feeding or resting. 

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 Australopithecuscould 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 though females were distinctly smaller.

The name 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 and likely slept in trees at night for security.
The ‘Genus’ Australopithecus is defined as a group of apes that had features between apes and ‘homo’. In other words they walked erectly.
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 propose splitting off some into other existing genera (or having additional genera). see here
This problem is one of the great challenges for archaeologists and anthropologists who classify specimens based (mostly) on morphology, habitat and behaviour.

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 all other apes, permanently 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.

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 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 variant subspecies. The main species of homo were polytypic.

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

The evidence for Homo is still fragmentary, especially in the distant past, but over time we have begun to understand that interbreeding of different-looking contemporary forms of Homo was much more common than we ever realised. 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 also after we found modern man interbred with Neanderthals and Denisovans when he reached Eurasia from Africa (more on this later).

With several subspecies of early man (capable of interbreeding). Each had particular mutations that might be an advantage ‘over all’ or in certain habitats that it could offer to the gene pool at large. Interbreeding, ensured that any differences that were advantageous could be combined and not lost forever.Such a situation is more common in evolution than once thought.

It also leads to the concept of chronospecies. A chronospecies is a species which slowly changes over time until it cannot be considered as 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 different environments causing such differences that eventually interbreeding becomes unlikely (e.g. loss of sexual attraction, mating rituals or physical differences) but natural selection can play a part.
This would be when the ‘in-between’ or older varieties become at a such disadvantage relative to the new variety that the benefits of the larger gene pool to overall adaptation of the species 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 very 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.

There is still considerable uncertainty and debate, but 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 archaeologists 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, 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 was one of the earliest species of ‘man’ (homo), 2.4 to 1.4 MYA.
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.

Sculpture H. Habilis, Élisabeth Daynès

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.

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, whether they were separate species or subspecies  (like H. rudolfensis, another contender for the title of being our ancestor).

The oldest homo remains, LD 350-1, (as yet unclassified into a species) is from Ethiopia 2.8–2.75 million years ago (mya) confirming that ‘homo’ evolved in Africa (in the savanna).

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

The title of 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 living of all human species, being present nine times as long as our own species, Homo sapiens.

H. Erectus was highly successful and mobile, spreading over Africa and Eurasia. 

It was H. Erectus that used improved stone tools, later tamed firefor the first time and undertook sea faring (colonising a few islands) .
It was H. Erectus that first began to hunt medium-large animals in groups (such as bovinesor elephants), moving to a diet richer in meat. ‘He’ would have had some tribes that routinely followed migrating herds.
With longer legs and relatively shorter arms (no longer climbing trees) and loss of hair, allowing running distances in the heat H. Erectus was bigger, faster and smarter (with an average brain capacity at least a third bigger than H. Habilis).

There is a conflicting need for the best pelvis for running on two feet and the ability to deliver babies with large heads. In ‘Modern man ‘ it is addressed by having children dependant on their parents for longer and more monogamous bonding in parents, allowing considerable brain growth after birth. Larger brains require a lot of energy and are best supported in hunter-gatherers by cooking and more meat in the diet with co-operative hunting of large animals.

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, had distinctly larger brains (up to 60% of H. Sapiens) than their smaller and earlier subspecies (or species).
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 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.

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