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| What Does DNA Tell Us About Our Position Among the Apes?? |
The new genetic data have substantially contributed to our understanding of the relationship between our species and its closest relatives. Based on several independent lines of evidence, we can now say with confidence that humans are more related to chimpanzees than to gorillas (Fig. 3). While the two species of chimpanzees are each other's closest relatives, their next closest relative is H. sapiens and not G. gorilla.
How do we know this? Evolutionary geneticists have been increasingly able to draw better and more robust inferences about the relationships among different organisms based on morphological and molecular genetic data, and new systematic methodology. These methods have also been used to determine our relationship among the apes. (See the Evolution and Phylogenetics unit.) In essence, groups of organisms (known as taxa) are placed into clades that are nested in larger clades based on shared ancestry. All of the taxa in a given clade are assumed to have a single common ancestor.
The first DNA-based data used to determine the relationships of the African apes came from mitochondria. These intracellular organelles enable animals to use aerobic respiration and have DNA that evolves relatively quickly in mammals. Consequently, mitochondrial DNA (mtDNA) is useful in analyzing the relationships of closely related species and populations within species. Mitochondria are also abundant in cells and, thus, mtDNA was easier to obtain than nuclear DNA.
New DNA amplification technologies developed during the 1990s, such as the polymerase chain reaction (PCR), made obtaining sufficient quantities of DNA much easier. (See the Genetically Modified Organisms unit.) Yet, for historical reasons, most taxonomic studies that used DNA characters were first done with mtDNA. Based on the evidence from mtDNA sequences, chimpanzees and humans were determined to be each other's closest relatives. These studies further suggest that humans and chimpanzees separated almost five million years ago, and the human-chimp clade separated from gorillas almost eight million years ago.
Critics raised an important point about the inferences based on the mtDNA studies: it is based on only a single, independently evolving gene region. When one considers very closely related groups of species, the constructed phylogenetic tree based on data from one gene may be different than one constructed from a different gene. Either one or both gene trees may not accurately reflect the true evolutionary history of the species. This phenomenon occurs because of genetic variation (polymorphism) in the ancestral species. Ancestral polymorphism can segregate differently in the different descendant species; that is, in one of the different descendant species, one of the variants may become fixed and in another descendant species a different variant may be fixed. Either natural selection or random genetic drift can cause this phenomenon. In either case, there is possibility that the history of the gene region may not reflect the history of the species. In other words, suppose that chimps really did split first from the lineage containing humans and chimps. It would still be possible that the phylogenetic tree based on a single gene may have gorillas splitting off from humans and chimps, or humans splitting from chimps and gorillas.
In the case of determining the relationships among the African apes, the solution to this challenge was simply the collection of more data from more genes. Mary-Ellen Ruvolo analyzed data sets from fourteen independent gene regions. In eleven of the cases, humans and chimps are each other's closest relatives (sister taxa). In two cases, gorillas and chimpanzees are sister taxa, and in one humans and gorillas are sister taxa. Statistical tests show that these results are highly unlikely to arise unless humans and chimpanzees are indeed each other's closest relatives. Subsequent analyses with even more genes have corroborated the conclusion reached by Ruvolo and the earlier mitochondrial DNA studies: we are closest to chimps.