How can human phylogeny be traced back to common ancestors?
Here is another one of my revision essays that I wrote for my evolutionary genetics module for my final year at university. As I said before, it was used for revision and therefore isn’t perfectly written or laid out. I apologise if detail is missing, there are typos or if it seems hard to follow.
Humans are closely related to primates, diverging approximately 6 million years ago. Fossil records aid in the picture of our evolutionary history, however there is still much controversy.
4 million years ago Australopithicus afaranses was believed to exist, exhibiting the first use of primitive tools. 1.7 million years ago Homo erectus was thought to have lived, walking upright and being more similar to modern humans. However, it is not known when/where it died out. 700kya there was a divergence, producing H. Neanderthalis and H. Sapiens (anatomically modern humans). Homo sapiens have been identified outside of Africa approximately 45 kya.
The picture of human evolution is complex as we do not know that these were direct common ancestors, it is possible that other ancestral species exist that we have not discovered. As a result, there is much debate as to where modern humans originated. There are, however, 2 main theories.
The out of Africa model suggest that modern humans emerged in Africa, before migrating out ~50kya. They are thought to have completely displaced other archaic human species by outcompeting them for resources.
Alternatively, it has been suggested that Homo erectus left Africa and populated the world. Next, modern humans evolved independently in different regions to populate the world. However, it is unlikely that it was possible for complex human traits such as speech to evolve independently in multiple different populations. Therefore, it has been suggested that there was gene flow between modern humans leaving Africa and archaic human species found in the rest of the world.
Phylogenetic trees of human populations have been constructed based on genetic distances measured by the fixation index of allele frequencies. These trees, based on protein polymorphisms, give support to the out of Africa model and the root of the tree in Africa.
The Y-chromosome of humans is specific to males. However, in the distant past it is thought that sex chromosomes were in fact autosomes. Gradually over time, one autosome took on specialised genes that specify male-ness. This chromosome, now specific to males, degenerated such that instead of 1000’s of genes it contains just 27. As a result of the genetic specificity of the Y chromosome to males, and the cultural feature of male surnames being predominant, the Y chromosome can be correlated with surnames. Additionally, the Y chromosome can be used as a population marker to trace historical migrations.
Phylogenetic trees can be constructed using information from the Y chromosome. As there is no crossing over within the chromosome, it is a faithful record of mutations that have occurred since its evolution as a sex chromosome. Different taxonomic units of these trees can be defined as haplogroups and can be identified as to where they are in the world.
The deepest branch of this tree is the megahaplogroup, containing no mutation on the Y chromosome. This can be located to a small group of individuals in Africa. The rest of the male populations derive from 1 mutation after this. Each previous haplogroup encompasses the next.
All males living outside Africa descend from 1 mutation. The next megahaplogroup, F, includes almost all men outside Africa. Haplotype C is the most common found in Australian aborigines and arose 40kya. There is evidence of a star-like diversification 50kya, indicative of a population expansion corresponding to migration out of Africa.
