Evolution and Complex Social Behavior

A changing family tree

Until recently, differentiation between species was based on identifying physical differences from the shape of bones and teeth.
Thers is no way of directly dating fossils, instead geological methods such as potassium argon dating are used to date the rock layer where finds occur.
Genetic analysis has transformed evolutionary studies.
- DNA analysis has shown that we share at least 98.7% of our genes with chimps and 98.4% with gorillas.
  - These differences arose over time through many genetic mutations.
  - Because these occur at a known rate, it means that we can more accurately estimate where ancestral line separate.
Hominids are members of a family of primates ometimes referred to as the 'Great Apes'.
A subgroup or tribe of this group are called hominins and include human and chimps.
Humans and their extinct ancestors are termed hominans with the term human applying to both our species ad extinct memvers of the genus Homo.

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The human lineage increasingly moved towards bipedalism, which may have been for easier movement between trees, for standing to pick fruit from trees or to transport food to a more dependant mate or infant.
Ardipithecus Ramidas, first found in Ethiopia in 1994, may be the earliest ancestor to be marked different from chimpanzee, and dates between 5.5 and 4.4 mya.
- It was omnivorous and showed very early signs of bipedalism.
Similarly, Orrorin tugenesis, found in Kenya in 2000 and dated to 6 mya, was the size of a chimp but it's leg bones suggested it walked upright.

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First discovered in the 1920's in South Africa, these were the first true bipeds and were a mix of human and ape attributes.
Lived between 3.9 and 3 mya in the grasslands of east Africa and had a brain size capacity of 390-545 cc.
Some bones had been adapted for walking upright, but mostly remained like an upright chimp.
Over the next 2 million years, lots of different species evolved, although the exact sequence is unclear.
Several different types existed at the same time to around 1.1 mya, and from one of these the Homo lineage developed.
Australopithecines are normally divided into at least eight types.
- e.g. Australopithecus africanus (3-2 mya)

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Homo habilis lived from 2.4 to 1.4 mya and was different from the Australopithecines in having a larger brain (500-800cc) and an anatomy clearly adapted to walking rather than climbing.
Its legs and feet were modern if rather short.
It is also the first fossil associated with stone tools and meat-eating.
These tools have been considered as one of the markers of more intelligent hominans.
Evidence from lithics found at Koobi Fora show thta the material they had been made of was brought from outcrops 3km away, showing the ability to plan.
- Tools might have developed out of a need to seek roots underground and scavende effectively.

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Until recently it was thought that only the homo genus was capable of making stone tools.
However, in 1996, animal bones showing butchering marks were found near a species of Australopithecines, changing this.
- It is now being argued that these were developed prior to homo habilis, and that they should be reclassified.
This helps to highlight the problem that so few remains have been discovered that timelines for different speries are only approximate.
- At least two species of Australopithecines co-existed with Homo habilis and considerable variation has been documented between habilis fossils.
The overall evolutionary sequence currently combines species from southern and eastern Africa but neither region has a complete sequence.

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Humans are differentiated from other species using several categories:
- Complex social behaviour
- Control of technology
- Creation of culture
Particular parts of these categories that make us different include the ability to communicate large amounts of knowledge, including abstract concepts, as this is absent in other modern species.
Another difference is food sharing; which chimps do not. This is thought to have developed as it was advantageous during times when food supplies were scarce.
Meat eating is also though to have been a major development in the evolution of more intelligent species.

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The coincidence of hundreds of lithics and broken animal bones, evidence of meat eating and larger brain size suggested to Leakey and Issacs that Homo habilis hunted and behaved socially more like modern humans.
- Habilis had a brain capacity up to 35% larger than the Australopithecines and may have been capable of basic speech.
- Issacs argued that sites at Olduvai and Koobi Fora were actually 'living floors' or 'home bases' and reflected hominins sharing food and working coorperatively.
Binford drew on ethnoarchaeological evidence of bone distribution from predator kills and experimental work on site formation processes affecting tools and bones in the region.
He suggested the sites might be palimpsests.
Microscopic analysis of the bones suggested that humans scavenged the meat off them after they ahd already been processed.
Today it is widely accepted that habilis might have scavenged kill sites but were not advance enough to establish and protect base sites.

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