The "evolutionary ladder" is a common misunderstanding about the nature of evolution. In its most common incarnation the ladder shows the evolution of humans from ape-like ancestors, but the idea may go as far as a great chain of being, from bacteria at the bottom up to humans, the "peak" of evolution.
The ladder is based upon the pre-Darwinian scala naturae, founded by Aristotle in a time when species were thought to be static essences created individually by God. The scala naturae organised both living and inanimate objects on a ladder, with Humans, or sometimes supernatural beings at the top.
The idea of the scala naturae was difficult to dispose of, and so was a stumbling block to those trying to understand Darwin's theory, causing misunderstandings such as a belief in teleological evolution. The belief that one existing, essential God-created species, purposefully evolves into another existing species, while special creation adds new simple micro-organisms at the bottom, to start their journey up to the top - humans, of course - was common initially.
There are a number of things which prop up the ladder:
- A belief that intelligence is the "peak" of evolution, and therefore all species can be organised on a ladder of intelligence (the legacy of hundreds of years of the scala naturae).
- Ambiguous phrases like "humans are evolved from apes". Humans are evolved from apes - or at least extinct animals that modern zoologists would classify as apes - just not contemporary apes like Chimpanzees (see Common descent). It would be just as inaccurate to say that Chimpanzees evolved from humans.
- The fact that some ancestral groups of animals are often anatomically very similar to some of their modern descendents, but not others (and we therefore assign them to modern groups), for example, the common ancestor of fish and amphibians is anatomically far more similar to modern fish species than modern amphibians, so it's convenient to say that "amphibians evolved from fish". Attenborough (1979) made a ground breaking television series by using modern groups to represent human ancestors, which led some viewers to believe not just in the ladder, but in a ladder whose purpose was to produce humanity.
- The use of the words "higher" and "lower" to describe different species.
The evolutionary ladder has never been a scientific concept, and Darwin talked of common descent, yet even this 1998 reprint of On The Origin Of Species shows a (rather giant) leap from a modern looking monkey to a reconstruction of a relatively recent possible human ancestor. Yet, despite the fact that the evolutionary ladder has never been a scientific theory, lay people seem to think that it is. When a fossil skull, named Toumaï¿½, was found in Chad in 2002 an article in Nature (Whitfield, 2002) which stated that the fossil prompted a rethink of human evolution was jumped on by creationists as the end of evolutionary theory altogether (Yahya, 2002). The reason was that when scientists were asked to comment about the fact that the find did not fit with the evolutionary ladder their response was that the evolutionary ladder is not a scientific theory and is baseless. The creationists jumped on this because to them the evolutionary ladder is evolution, or even if they don't think that, it's still convenient to pretend that the evolutionary ladder is the prevailing evolutionary theory when there are scientists attacking it.
- Attenborough, D., 1978. Life On Earth, London: BBC/Collins books.
- Whitfield, J., 2002. Oldest member of human family found, in Nature 2002/Jul/11 
- Yahya, H., 2002. New Fossil Doscovery Sinks Evolutionary Theories, Islam Online .
- Colby, C., 1996. Introduction to Evolutionary Biology. Talkorigins.org, .
- Dennett, D., 1995. Darwin's Dangerous Idea, Simon & Schuster. Discusses pre-Darwinian essentialist ideas about kinds.
- Mayr, M., 2001. What Evolution Is, BasicBooks. Ch 1 debunks the scala naturae and teleological evolution.
- Gould, S.J., 1990. Wonderful Life, Hutchinson. Begins by describing the ladder and teleological evolution, and then shows that evolution is blind and purposeless.
See also: Common Descent