Biology Reference
In-Depth Information
4
Genes and gene families
4.1 The origins of human genes
Certainly the growth of the forebrain has been a success:
He has not got lost in a backwater like the lampshell
Or the limpet; he has not died out like the super-lizards.
His boneless worm-like ancestors would be amazed
At the upright position, the breasts, the four-chambered heart,
The clandestine evolution in the mother's shadow.
W.H. Auden. In Time of War, Commentary from
Journey to a War
(1939)
Some human genes appear to have originated comparatively recently. Indeed a
very few have emerged in the relatively short space of time that has elapsed since
divergence from chimpanzee some 7 Myrs ago. Other human genes appear to have
homologues in simple organisms implying a very ancient origin, in some cases
even predating the divergence of prokaryotes and eukaryotes. In the following
sections, representative examples of genes are cited in order to illustrate the enor-
mous variability in the antiquity of extant human genes. To some extent of course,
the origin of a given gene is a matter merely of semantic distinction. Thus
Doolittle's (1992) truism stated: 'new proteins come from old proteins as a result
of gene duplications followed by base substitutions.' This notwithstanding, it has
still been possible to draw up a working classificatory scheme for proteins based
on their 'apparent invention time' (
Table 4.1
).
4.1.1 Genes with a specifically human origin
There are relatively few known examples of human genes which originated in the
last 7 Myrs after the divergence of the human and chimpanzee lineages. One such
gene is the potentially functional immunoglobulin V
gene located 1.5 Mb telom-
eric to the human C
gene which is putatively human-specific (Huber
et al
., 1994).
Further studies of the immunoglobulin
κ
gene segments on the short arm of human chromosome 2), have provided evidence
for the human-specific partial duplication of the locus (Ermert
et al
., 1995). The
duplicated portion of the immunoglobulin
locus (comprising one C
, 5 J
κ,
and 76 V
genes and
pseudogenes, is not found in the chimpanzee or gorilla. Thus, the duplication event
must have occurred after the branchpoint of humans from the great apes. Huber
et
al
. (1994) estimated that this duplication event occurred 1 Myrs ago.
locus, which contains 36 V
