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Resetting the "molecular clock hypothesis"
Riv Biol. 2008 Jan-Apr;101(1):93-108.
Ancient fossil specimens of extinct species are genetically more distant to an
outgroup than extant sister species are.Huang S.
The Burnham Institute for Medical Research,10901 North Torrey Pines Roads, La
Jolla, CA 92037.
There exists a remarkable correlation between genetic distance as measured by
protein or DNA dissimilarity and time of species divergence as inferred from
fossil records. This observation has provoked the molecular clock hypothesis.
However, data inconsistent with the hypothesis have steadily accumulated in
recent years from studies of extant organisms. Here the published DNA and
protein sequences from ancient fossil specimens were examined to see if they
would support the molecular clock hypothesis. The hypothesis predicts that
ancient specimens cannot be genetically more distant to an outgroup than extant
sister species are. Also, two distinct ancient specimens cannot be genetically
more distant than their extant sister species are. The findings here do not
conform to these predictions. Neanderthals are more distant to chimpanzees and
gorillas than modern humans are. Dinosaurs are more distant to frogs than
extant birds are. Mastodons are more distant to opossums than other
placental mammals are. The genetic distance between dinosaurs and mastodons is
greater than that between extant birds and mammals. Therefore, while the
molecular clock hypothesis is consistent with some data from extant organisms,
it has yet to find support from ancient fossils. Far more damaging to the
hypothesis than data from extant organisms, which merely question the constancy
of mutation rate, the study of ancient fossil organisms here challenges for the
first time the fundamental premise of modern evolution theory that genetic
distances had always increased with time in the past history of life on Earth.