II. The first four billion years.

III. Primate evolution.

IV. Hominoid evolution.

Terms

Note Page 440, Fig. 18.14: "Hominid fossil sites..." should read "Hominoid fossil sites..." Hominids did not appear until later. (See text.)

I. Phylogeny refers to the evolutionary origins and relationships of species, including how closely they are related to each other and to ancestral species. There are several major tools for reconstructing phylogeny.

A. Fossils are cells and tissues that have become infiltrated with minerals, preserving the form of the original. For the most part, only the bony parts of animals are preserved, although occasionally impressions of other parts are preserved. Virtually all of our knowledge of the forms of extinct organisms is based on fossils, which can survive billions of years. Even though fossils may be only a fragment of the original organism, much can be learned about the form and function of the organism.

B. DNA sequences allow us to establish affinities among existing species (and occasionally recently extinct species). Genes that occur in all life forms, for example, genes that code for enzymes associated with DNA replication and energy utilization, have similarities that reflect common origins. The greater the similarity of DNA sequences, the more recently the compared species have diverged from a common ancestor. Very closely related species may have very similar DNA sequences, especially in coding regions of genes.

C. Amino acid sequences in proteins provide information on phylogeny in much the same way that DNA sequences do. As in the case of DNA, amino acid sequences are helpful in studying the relationships of the most distant life forms.

D. Gene organization is sometimes very informative. By gene organization is meant the presence and location of introns, the presence of functional domains in the protein product, etc.

E. Chromosomal structure and banding is useful in comparing eukaryotes, although chromosome structure changes rather rapidly on the evolutionary time scale. It is useful primarily for comparing closely related species. Chromosomal mapping of specific genes is extending the usefulness of chromosomes by allowing reconstruction of the chromosome rearrangements that occurred during evolution.

F. Allele frequencies at polymorphic loci are useful in establishing relationships of populations within a species but not for comparison of different species.

A. The earth is ca. 4.5 billion years old.

B. The earliest fossils are of bacterium-like cells that lived some 3.5 billion years bp (before present). Such cells already had a very complex metabolism, with DNA replication and transcription, the formation of many enzymes, etc.

C. The genetic repertory is thought to have expanded by chance gene duplication and divergence, with favorable mutations displacing previous alleles.

1. Genes with essential function, such as DNA polymerase, must have existed and been preserved for a very long time. Nevertheless, one can see similarities of structure between these genes in diverse species, even though they have evolved separately for billions of years.

2. Genes that have duplicated and diverged more recently constitute gene families that have many similarities. Examples: globin genes, immunoglobulin genes.

D. The evolution of multicellular organisms (e.g. animals and plants) began only about 600 Myr bp.

A. There was a remarkable diversification of mammals beginning about 65 Myr bp, when most of the present Orders were formed. The first primates appeared at that time.

B. Major evolutionary steps are reflected in living primates.

1. The prosimians consist of
a. Tree shrews, the first to diverge from the primitive primate lineage;

b. Lemurs and lorises;

c. Tarsiers.

2. The anthropoids consist of all other primates. The most divergent from humans are:
a. The New World monkeys, distinguished by prehensile tails;

b. The Old World monkeys, e.g. baboons, macaques.

3. The hominoids consist of apes and humans.
a. The apes that are most divergent from humans are the siamangs and gibbons of Southeast Asia. They are small and move primarily by brachiation (swinging by arms).

b. The great apes are our closest relatives. These include orangutans (genus Pongo), the most distant of our close relatives, gorillas (genus Gorilla), and chimpanzees (genus Pan), our closest relative.

c. Homo sapiens is the only living hominid.

A. About 15 million years ago, the lineage that produced modern orangutans separated from the common hominoid ancestor. Among the lines of evidence that support this are chromosome banding patterns, with humans/chimpanzees/gorillas forming a similar group that is distinct from the orangutan pattern.

B. The common ancestor of the gorilla/chimp/human lines was a small ape. There are several fossil candidates for this ancestor, but the identity is not certain.

C. The gorilla/chimp/human splits occurred at about the same time, some 5-7 million years ago, but the DNA differences between human and chimp are a bit less than human/gorilla and chimp/gorilla, indicating that the gorillas separated first from the human/chimp lineage, followed by later separation of humans and chimps from each other. Comparison of DNA sequences among these three species shows them to be ca. 99% identical, about the same as between grizzly bear/polar bear or between horse/zebra.

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