Deep time refers to an enormous timescale with which geologists and scholars of evolution must deal. Homo sapiens first emerged as a separate species, not 800 years ago, or even 10,000 years ago, but somewhere between 100 and 200 thousand years ago. The earliest identifiable human ancestor appeared between 7 and 6 million years ago. And the earliest primates occurred between 65 and 55 million years ago. The planet itself if 4.6 billion years old.
These timescales can be hard to get your head around but they are pivotal to understanding the overall story of life on earth.
Physical evidence of human ancestors (and other forms of prehistoric life) comes primarily from the fossil record. Fossils consist of the remains of once-living organisms that have been preserved through a natural process whereby the organic tissue is replaced with minerals from the surrounding environment. In short, they turn into rock.
Fossilization is a very selective process.
Only a small fraction of living organisms become fossilized. Conditions that lead to fossilization are just too rare.
Second, even when fossilization occurs, often not the whole creature becomes fossilized. It is technically possible for soft tissues to be fossilized, but it is extraordinarily rare. Far more often, what we find are the larger and denser bones of the body. Thus for human ancestors, we have found lots of teeth and skulls, along with some long bones and pelvises, but very few ribs and bones from the hands. These bones are just too small and fragile to regularly withstand the fossilization process. An additional challenge to the fossil record is that paleoanthropologists have to actually find the fossils!
Paleoanthropology has found a powerful new source of information—Genetic Analysis! The Mitochondrial Eve study.
Mitochondrial DNA mtDNA is found only in your mitochondria (small organelles inside of your cells). Replacement Thoery
Out of Africa Theory...