text: Missing link is a term used by non-biologists, especially by young Earth creationists who deny evolution, to refer to an alleged "gap" in the fossil record. The term is usually applied to an alleged gap in the fossil record between humans and other primates. For example, Newsweek magazine in a note about "Ardi" (a 4.4 million-year-old hominid fossil) commented: "Scientists getting closer to finding the missing link" (October 12, 2009; p. 12). No. Scientists aren't looking for a missing link between humans and other apes. The idea of such a fossil originated in the 19th century and has been abandoned by all but journalists and those ignorant of modern evolutionary theory (not that the two are mutually exclusive).
  The notion of an ape-man fossil existed among scientists well into the 20th century, as is evidenced by the Piltdown hoax. It is an amazing and surprising fact that we now have "a rich supply of intermediate fossils linking modern humans to the common ancestor that we share with chimpanzees" (Dawkins 2009: 150).
  At the time Darwin published his Origin of Species (1859) the fossil record was sparse, and ways of dating what fossils had been discovered were primitive. It is a testament to good luck and hard work that we now have an extensive fossil record in certain areas. Fossilization is "a highly improbable event, and most creatures that have ever lived do not become fossils" (Prothero 2007: 51).
  Consider the chain of events that happen to an organism after it dies. First, there are the biological agents (bacteria, fungi, insects, and other decomposers, scavengers, etc.) that break down or destroy an organism after death. The soft parts of animals decay or are eaten quickly, so they almost never fossilize. Only the hard parts, the shell or skeleton, have a reasonable chance of preservation. After an animal dies, its bones are typically scavenged and broken, so little or no remnants of the skeleton may actually survive....
  In the marine realm, there are many agents of destruction as well. (Prothero, 51-52)
  If a shell or bone survives the ravages of time, predators, geological changes, and natural dissolution, it must still escape destruction by erosion if it is to become a fossil. The odds that any organism will end up as a fossil are miniscule. Donald Prothero estimates that "the total number of species that are represented in the fossil record is a tiny fraction of 1 percent" (ibid. p. 52).
  Yet, despite all the odds against it, "the fossil record of extinct humans is now incredibly rich, so there are more 'discovered links' than there are 'missing' links (Prothero 2007: p. 126; chapter 15 of Prothero's Evolution covers the human fossil record in detail, as does chapter 7 of Dawkins's Greatest Show on Earth.)
  The concept of missing links is related to an ancient philosophical notion called the Great Chain (or Ladder) of Being, which depicts a god above the top as the creator of everything else which is depicted as starting with various pure spirits and moving down the line to humans (bodies and spirits) to other animals (bodies but no spirits, yet animate) to plants (animate, but no locomotion) to inanimate objects. Some philosophers, like Plotinus, put a god above the chain and non-being below it. Each link on the chain represents a place in a hierarchy of beings. The idea of a divinely ordered hierarchical creation was used to justify many pernicious practices such as sexism and racism, but here we are concerned only with two misconceptions affecting ideas of evolution. The hierarchy notion implies that humans are a "higher" form of creature than all those below us on the ladder. The ladder or chain image implies that each rung or link is intermediate (or "transitional") between the ones above and below it and that there should be a form of creature between humans and the other primates (the so-called ape-man missing link). Both notions are false.
  As everybody who understands evolutionary biology knows, the metaphor of the ladder of being is grossly misleading because evolution has not worked by one species giving birth to another species and so on down the line from one-celled creatures to humans. This gross model of evolution is, however, one that many creationists have, as is evidenced by the kinds of questions they ask of evolutionary biologists. If humans evolved from monkeys, why are there still monkeys? Where is the fossil for the crocoduck? Why don't monkeys give birth to humans once in a while? Why aren't humans (and other species) still evolving? (We are!)
  Darwin's metaphor for how species evolved from common ancestors is that of a tree with many branches, but all metaphors eventually become misleading. Species evolve gradually, sometimes rather quickly, sometimes rather slowly, through processes of natural selection at work over millions of years. Every species can be seen as a "missing link" or a "transitional creature," but it is foolish to look for the half-ape half-human creature or the half-crocodile half-duck creature. What isn't foolish is to look for fossils of species that lived before amphibians evolved but after fish had evolved, which is what a team of scientists including Neil Shubin did. Based on their knowledge of the history of evolution and fossilization, they chose to look in a rocky area of the Canadian Arctic that dated from the late Devonian period.
  Shubin and his colleagues found "Tiktaalik on Ellesmere Island, some 600 miles from the North Pole, in deposits dating to 375 million years ago. Like all fish, Tiktaalik possesses fins and scales. But it also has a number of distinctly un-piscine characteristics, including a neck, a flat, crocodile-like skull, and robust ribs. Tiktaalik neatly fills the gap between previously known tetrapod-like fish such as Panderichthys, which lived some 385 million years ago, and the earliest tetrapods, Ichthyostega and Acanthostega, which lived about 365 million years ago. 'Tiktaalik blurs the boundary between fish and land animals,' said Shubin. 'This animal is both fish and tetrapod; we jokingly call it a fishapod.'"*
  It should go without saying that Tiktaalik is not a "missing link" in the sense that its parents were a different species from it. No, its parents, like every set of parents who has ever existed, were members of the same species as their offspring.
  If you are now asking Why are we not still one-celled animals? then you don't know how evolution works and should read one of the books listed in the section below. If you think that fossil evidence is the only or the best evidence for evolution, you don't understand the evidence for evolution and should read one of the books listed below.
  See also Archaeoraptor, the Cambrian "explosion," creationism, Reiner Rudolph Robert Protsch (von Zieten), A missing link? Fishing for Jesus (about the discovery of Tiktaalik), Media Junk (about "Ida," a 47-million-year-old fossil), and my review of The Greatest Show on Earth: The Evidence for Evolution by Richard Dawkins.
  further reading
  books
  Coyne, Jerry A. 2009. Why Evolution Is True. Viking Adult.
  Dawkins, Richard. River Out of Eden: A Darwinian View of Life (1995, Basic Books).
  Dawkins, Richard. Climbing Mount Improbable (1996 Viking Press).
  Dawkins, Richard. The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design (1996: W.W. Norton).
  Dawkins, Richard (2004). The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution. Houghton Mifflin.
  Dawkins, Richard. 2009. The Greatest Show on Earth: the Evidence for Evolution. Free Press.
  Lovejoy, Arthur Oncken. 1936. The great chain of being: a study of the history of an idea. (Google books.)
  Prothero, Donald R. Evolution: What the Fossils Say and Why It Matters. (Columbia University Press, 2007).