With the development of tools to parse the sequence of nucleic acids in genes, the key sequences have been studied in bacteria, plants and animals. Genes essential to early development of the embryos of multicellular plants and animals are shared by essentially every species tested. Complicated as we consider man to be, we nevertheless share long sequences of DNA with the simplest of creatures, the bacteria. Additional genes provide for the body structure and metabolic needs of a particular species.

figure 6

Darwin had proposed that the development of new species in evolution would involve branching again and again over time. In developing his diagram he theorized that the species that initiated a branch might become extinct and be represented by only fossil remains. Paleoentologists became expert at developing the family trees of existing species from the fossil record. These fill in specific details in a Darwinian diagram. A favorite example is the horse.

The further back in time one goes the less satisfactory fossils are, because of the large gaps that develop. The gaps are particularly large when one goes back to soft-bodied animals that have left no fossils. However, our newfound ability to sequence the entire genome of a species provides a powerful tool to discern evolutionary relationships. The degree of correspondence between the DNA sequences of species describes evolutionary relatedness.

figure 7

A broad based tree divides life into three classes: the Bacteria, all microscopic and without a definable nucleus; the Archaea, defined as separate class of life in the past 30 years, also microscopic but fundamentally different from bacteria; and eukaryotes, organisms with their DNA gathered into a nucleus.

A considerable portion of eukaryotes are single celled and microscopic but eukaryotes (eukaryote simply means true nucleus) also include all multicellular animals and plants. Geneticists are developing complete (or near-complete) DNA sequences of more and more organisms. A recent diagram of all species that have been sequenced so far shows the family tree of life, as we currently know it. Archea are in green, bacteria in violet, and eukaryotes in red. The heavy emphasis on bacteria is because they are easy to sequence, with no more than 4,000 or 5,000 genes. Man and his near relative, the chimpanzee, with around 30,000 genes, are on another branch. Not too far away is the mouse, but yeast and the causative agent of malaria are also on the same branch.

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