Re: Change from 48 to 46 chromosomes

David J Betty (
24 Oct 1995 01:53:15 GMT

Concerning ape karyotypes, donald e. tyler wrote:
: Any evolutionary guesses
: as to how apes with 48
: chromosomes were changed
: to homo sapiens with 46?

48 chromosomes appear to be an ancestral state for great apes. Sometime in
the human line of descent between the last common ancestor of humans and
(presumably) chimps, two pairs of acrocentric chromosomes evidently fused
to become a pair of large metacentric chromosomes. This pair are the
second largest in humans, and examples are labelled chromosome 2. (Human
chromosomes are labelled in approximate descending order of size.) The two
halves of human chromosome 2 (2p and 2q) each have separate chromosomes as
homologues in the karyotypes of chimps.

This particular type of chromosomal mutation is called Robertsonian
translocation or Robertsonian fusion. To become fixed in a population, the
mutation may well have been associated with an epsiode of speciation.

There is an extensive literature on the role of chromosomal variation and
speciation. However, because there have probably been multiple episodes
of speciation during the last 5 million years of our evolution, it is not
feasible to be more precise about the timing.

A useful reference to begin reading about ape chromosomes is:
Yunis JJ & Prakash O, (1982) "The origin of man: a chromosomal pictorial
legacy." Science, 215: 1525-1530

A superb, hardcover monograph on chromosomes and evolution, which should
be available in university libraries, is:
White MJD (1973) Animal cytology and evolution. (third edition) Cambridge
University Press

David John BETTY ( tel: Australia (0)6 249-4715
Human Genetics Group, Clinical Sciences Division,
John Curtin School of Medical Research, Australian National University,
G.P.O. Box 334, Canberra, A.C.T. 2601, Australia