"Missing link" is a term for a transitional form from the fossil record that connects an earlier species to a later one, or which connects two different species to an earlier ancestor. The missing link may have features common to both species; for example an early avian fossil could have feathers much like a modern bird but still retain the bony tail and teeth of a dinosaur. This would be considered a missing link between dinosaurs and birds. The term "missing link" often refers specifically to an animal representing an evolutionary transition from monkeys or apes to modern man. This usage is based on a misunderstanding of basic evolutionary theory, as biologists do not propose that such a link exists; rather, man has a common ancestor with other primates.
The missing link is a popular and not a scientific concept. Scientists studying the fossil record have long known that not every species that lived was 'lucky' enough to leave behind a fossil. More importantly, populations are constantly changing and species are statistical constructs and not ideal-types; therefore, there is not scientific meaning to the notion of a "transitional form."
This being said, a number of fossils exist that do indicate a link between earlier and later forms of animal. The lobe-finned fish Eusthenopteron is thought to be the first step towards land-dwelling amphibians; fossils of feathered dinosaurs in China seem to indicate that feathers were commonplace even before true birds evolved; and recently reexamined fossils of some amphibians have shown that some bones of these animals bear striking resemblances to those of fish.
External links
Hominid Species (http://www.talkorigins.org/faqs/homs/species.html) at talkorigins.org
13 million year old "missing link" found in Spain (http://news.bbc.co.uk/2/hi/science/nature/4014351.stm) (BBC)
In biology, the theory of universal common descent proposes that all organisms on Earth are descended from a commonancestor or ancestral gene pool.
This theory is now generally accepted by biologists, and the last universal commonancestor (LUCA or LUA), that is, the most recent commonancestor of all currently living organisms, is believed to have appeared about 3.5 billion years ago (see: origin of life).
One such tree showing the paths of descent from a commonancestor is depicted in the article on phylogenetic trees.
The most recent common patrilineal ancestor of any two males, and the most recent common matrilineal ancestor of any two individuals can be determined by genealogical DNA tests.
The most recent common patrilineal ancestor of all living male humans, known as Y-chromosomal Adam, and the most recent common matrilineal ancestor of all living female humans, known as Mitochondrial Eve, have been established by researchers using tests of the same kinds of DNA as for two individuals.
This concept is described in Richard Dawkins' book, The Ancestor's Tale, in which he imagines a backwards 'pilgrimage' in time, during which we humans 'meet' all the other species of organism with which we share a commonancestor.
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