We Eukaryotes

Part 5 - Hominids

After splitting away from the chimpanzees, 5.8 million years ago, hominids slowly evolved larger brains.

Australopithicus (which lived between 4 million and 2 million years ago) had a brain measuring 450 cm3 (cubic centimetres), not much different from the modern chimpanzee's brain of 300 to 500 cm3.   Australopithicus began eating meat more than 3 million years ago probably because plant food was not always available. Initially, this was probably insects and small animals possibly killed by fire, starvation or other predators. To survive, they were forced to become scavengers as well as hunter-gatherers.

Meat has more calories and protein than plant foods but carnivorous animals need to range farther to find prey and must use more energy to hunt them down. The apelike proportions of the australopithecines, slowly began to morph into a long-legged body built for sustained running which gave hunters more success with larger prey and also helped them evade large predators. Standing upright also made them appear larger as a deterrent to predators while also improving distant vision as their eyes were at a higher level.

Homo habilis, (which lived between 2.8 and 1.5 million years ago) evolved a brain volume of 600 cm3 and left evidence of the use of stone tools. The rapid brain growth in infants permitted language acquisition and learning perhaps as early as 2 million years ago.

Homo habilis also evolved the many skeletal changes permitting them to move easily on two legs. These changes were not from choice. They were driven by a drastically changing way of life, but this occurred over a 2 million year period and the incremental changes would not have been obvious to anyone living at this time.

The global climate began a long cooling trend about 3 million years ago possibly because of a reduced radiation output from the sun. The most recent Ice Age, the Pleistocene Epoch, began about 2.6 million years ago and lasted until about 11,700 years ago. During this time glaciers spread over huge parts of the planet including all of Canada and Greenland, most of Britain and northern Europe and parts of the USA. The climate became much colder and drier as much of the water became locked up in ice. Global temperatures averaged 5 to 10 degrees C (9 to 18 degrees F) cooler than today's temperatures.

The huge glaciers removed much of the water on the planet, severely reducing sea level and rainfall, which was about half of what it is today. This drastically affected East and Central Africa, where Homo habilis was just evolving.

The tropical forest environment slowly disappeared as it changed to open savannah grasslands. The fruits, leaves, tubers and seeds the australopithecines and Homo habilis ate became scarcer and only available seasonally. Water sources were less reliable also, so our ancestors were forced to forage over a much larger area in search of water and edible plant foods. Hominids could no longer hide from large predators in trees, which were fewer and far between. They were forced to walk or run for long distances and also to cooperate with each other for hunting and defence.

Walking on two legs freed the hands for carrying food, children and tools and reduce the energy needed for walking and running. Arms became shorter, making it easier to run and, as hominids no longer could live in trees, they no longer needed the toes to grip branches and the big toe became aligned with the other toes which made running easier. The skull move further back on the spine to balance the weight of a heavier brain, knee and ankle joints became stronger, the spine became S-shaped and lower vertebrae larger, to support more weight.