Human Evolution

Tibor Cemicky

Tracing Human Evolution

• Our understanding of human evolution is based mostly on fossils

• Hominid fossil record contains many gaps• Only a tiny proportion of animal bodies became

fossilized (most was eaten by detrivores, decomposed by bacteria or broken down chemically)

• Hominid fossils consist of bones and teeth only• These remains were preserved because they

were covered by dry sediments

Problems with tracing human origins and evolution

• Because there are gaps in the hominid fossil record it‘s hard to say how different species of hominid are related

• Many details of human evolutionary origins are also uncertain

• Discoveries of small number of fossils can cause major changes in the prevailing theories

Dating Fossils• To place fossils into a sequence it is necessaey to know

their dates• To find these dates we use radiotopes (radioactive

isotopes of chemical elements)• When an atom of a radioisotope decays, it changes into

another isotope and gives of radiation• The rate of decay varies between different radioisotopes

and is called the half-life • Half life is the time it takes for the radioactivity to fall to

half of its original level• The two most commonly used radioisotopes for dating

fossils are 14C and 40K • Half-life of 14C is 5370 years and half-life of 40K is 1250

million years

Hominid Diets and Brain Size• The brains of the early hominids

(Australopithecus) were only slightly larger than the brains of apes

• Their strong jaws indicate vegetarian diet• About 2.5 million years ago Africa became

much cooler and drier and forest was replaced by savannah grassland

• This change of habitat might be the reason behind the evolution of the first species of Homo

• Development of increasingly sophisticated tools led to a change in diet that included meat obtained by hunting

• This change in diet corresponds with the start of increase in brain size

Increase in Brain Size

• The increase in brain size

can be explained in two ways:

– Eating meat increased the supply of protein, fat and energy making it possible for the growth of larger brains.

– Catching and killing prey on the savannas was more difficult than gathering plant foods so natural selection favoured hominids with larger brains and greater intelligence

Brain Size Evolution

Genetic and Cultural Evolution

• The large brain of Homo sapiens and other species of Homo allows much to be learned during both childhood and adulthood

• Language, tool making skills, hunting techniques, methods of agriculture, regilion and many other forms of behaviour are passed from generation to generation by teaching and learning

• New methods, inventions or customs can be incorporated into what is passed on.

• This process is called Cultural evolution and is different from Genetic evolution

Cultural vs. Genetic Evolution

• Cultural evolution does not involve changes in allele frequencies in the gene pool

• Changes due to cultural evolution can happen during human life time, genetic evolutions happens over generations

• Cultural evolution involves characteristics aquired during a person‘s life, genetic characteristics are inherited

• Cultural evolution is responsible for most of the changes in the lives of humans in the last few thousand years

• Genetic evolution has been reduced recently due to cultural evolution (e.g medicine restricts natural selection)