Molecular Phylogeny of Primates

Molecular phylogeny of Homo and other Primates

All members of the order Primata are characterized by a distinctive skull feature, a complete bony ring around the eye orbits that facilitates binocular vision. The diagram shows the relationships of modern and extinct species of Homo to other living primates, as inferred from the 13 protein-coding genes (~ 12 Kbp) of the mitochondrial DNA (mtDNA) genome.

The evolutionary tree shows the evolutionary relationships of humans (genus Homo) relative to the other Great Apes, the Great Apes relative to the Lesser Apes, Apes relative to the Old World Monkeys, the separation of Old and New World Monkeys, and the relationships of "Simian" or "monkey-like" primates to Pro-Simian tarsiers. Another pro-simian group is the Lemurs (not shown).

In such trees, degree of evolutionary relationship is determined by tracing the branching order back to the Most Recent Common Ancestor (MRCA) of any group relative to another. Organisms that share a more recent MRCA are more closely related. The degree of evolutionary similarity is determined by counting the number of base changes (Single Nucleotide Polymorphisms, or 'SNPs') [numbers above the branches] along the branches connecting any two species. Among the Great Apes, humans (Homo) are more closely related to chimps (Pan) than either is to gorillas (Gorilla) or to orang-utans (Pongo). It also shows that extinct and modern human species are far more similar to each other than are different species of chimps, gorillas, or orang-utans. [Numbers in bold below the branches indicate the statistical confidence in the relationship, calculated in a manner to be discussed].

Homework: Prior to the advent of molecular data in the 1960s, it was assumed that the perceived large organismal differences between Humans and other Great Apes were due to accelerated genetic change along the human lineage. Organismal and genetic differences between the 'Ape-like' Chimps, Gorillas, and Orangs were expected to be relatively minor. Test this hypothesis by counting & recording the number of SNP differences between the six pairwise combinations of Homo sapiens, Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus. For each pairwise comparison, count the lengths of all of the horizontal branches connecting them. For example, the difference between Homo sapiens and Homo denisova is 8.9 + 1.4 + 7.6 + 9.9 = 27.5 . Record the results in the bottom half of a 4 x 4 (Excel) table. Answer the following questions.

(1) What is the expected result if the pre-molecular hypothesis is correct? State this as a hypothesis. Do the data support or reject the hypothesis? Explain.

(2) Do the data support or reject the hypothesis that morphologically similar chimps and gorillas are also more genetically similar to each other than to humans? Explain

(3) Repeat the calculation for a New World monkey (Ateles), Old World monkey (Papio), Lesser Ape (Hylobates), and Neanderthal (Neanderthal). Are Monkeys (with tails) more similar than Apes (without tails)