Human Evolution

Relationships among Humans and Extant Apes

Primate taxon Catarrhini includes...

Apes

including African Great Apes

are distinguished by...

Old World monkeys

relatively large brains

absence of a tail

a more erect posture

Greater flexibility of the hips and ankles

increase flexibility of the wrist and thumb

changes in the structure of the arm and shoulders

including Homo sapiens

first proposed by Thomas Henry Huxley 1863

supported by molecular analysis

distinguished from other apes by the synapomorphies...

elongated skulls

enlarged brow ridges

changes in the front of the upper jaw

fusion of certain bones in the wrist

enlarged ovaries and mammary glands

reduced hairiness

Sarich and Wilson's phylogeny of the apes 1967

Experimental steps:

  1. Took purified human serum albumin and injected it into rabbits.
  1. Waited for rabbits to make antibodies.
  1. Took blood serum from rabbits, which at that point contained rabbit antihuman antibodies.
  1. Mixed the rabbit serum with purified serum albumin from a variety of apes and Old World monkeys
  1. Used the strength of the immune reaction as a measure of similarity among the albumins they tested
  1. Assumed the similarity of two species' serum albumin proteins reflects the species' evolutionary kinship

Shows that humans are close kin to gorillas, chimps, and bonobos

Possible Phylogenies of humans and the African great apes

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Molecular Evidence:

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Molecular Evidence:

Insert: Genome of the retrovirus HERV-K

Incomplete lineage sorting...

phylogenies of genes and the phylogenies of species are not necessarily the same

Morphological Evidence:

Most analyses have produced trees in which humans and the chimpanzees are closest relatives

uses DNA sequences from a variety of loci, especially mitochondrial gene COII

different descendant species lose different ancestral alleles, such as in HERV-K, so it is better to use many independent genes

Morphological Evidence:

occurs when common ancestor is polymorphic and different alleles are preserved in different scenarios

suggests humans diverged from the lineage before it acquired the HERV-K insertion unique to gorillas and chimpanzees

A MINORITY of analyses

Humans lack some traits that the others possess, especially skeletal traits associated with knuckle walking

Characteristics of the skulls of the great apes

Resolution of the human/chimp/gorilla evolutionary tree on morphological grounds depends on the identification of which traits are ancestral vs derived

Dryopithecus (extinct european ape) shares several cranial traits with gorillas that are absent in chimpanzees and humans

some unique cranial traits in gorillas now considered to be ancestral

more detailed morphological analyses also suggest that humans and chimpanzees are closest relatives

Genetic Differences between Humans, Chimpanzees, and Gorillas

most obvious genetic difference between humans and the African great apes is in their karyotypes

Gorillas and chimpanzees: 24 pairs of chromosomes

Humans: 23 pairs of chromosomes

sometime after our lineage split from that of chimpanzees, two chromosomes fused to become what we now know as chromosome 2

Overall divergence between humans and chimps is a little over 1%

Size of genome (human): ~3 billion

~ 35 million snps

Assortment of chromosomal rearrangements

Proteins...

~29% of the proteins encoded in our genome are identical to the homologous protein in chimps

In nonidentical proteins, typical difference is two amino acid substitutions

Humans and chimpanzees are more similar to eachother than even mice and rats

Hominin Lineage

the line to hominins began between 6 milion and 8 million ya, at which point we last shared common ancestors with living bonobos and chimpanzees

paleontologists have not yet identified fossils of the last common ancestor that we share with our closest living relatives