Biology Reference
In-Depth Information
Delta T
50
H
7
6
5
4
3
2
1
0
Pygmy chimpanzee
= 0.7
N = 9
Chimpanzee
= 1.6
N = 75
Human
= 2.3
N = 151
Gorilla
∆
= 3.6
N = 83
Orangutan
∆
= 4.8
N = 85
Gibbon
= 7.3
N = 107
Old World monkeys
Figure 2.4.
Phylogeny of the hominoid primates as determined by average linkage
clustering of delta T
50
H values derived from DNA-DNA hybridization (redrawn from
Sibley and Ahlquist, 1987).
(Ruvolo
et al
., 1991; Horai
et al
., 1992) and a polymorphism at the
HLA-DQA
locus (Gyllensten and Erlich 1989). Studies of immunoglobulin
pseudogenes
have been equivocal (Ueda
et al
., 1985, 1989). Taken together, it is clear is that the
split between humans, chimps and gorillas was very close and that it is not unrea-
sonable to expect that different sequences will have diverged to variable extents in
the different lines. Thus the importance of the recent data of Kumar and Hedges
(1998) discussed in Section 2.2. These data yielded slightly revised divergence
times for higher primates with tighter error margins: gibbons, 14.6 ± 2.8 Myrs
ago; orangutan, 8.2 ± 0.8 Myrs ago; gorilla, 6.7 ± 1.3 Myrs ago, chimpanzees-
humans, 5.5 ± 0.2 Myrs ago.
The rates of accumulation of mutations appear to vary by as much as seven-fold
between different primate lineages (Koop
et al
., 1989). The line of descent from
the primate node to humans shows a slowdown in evolutionary rates from 7.7 ×
10
-9
fixed changes site
-1
year
-1
for the first 15 Myrs (55-40 Myrs ago) to 1.3 × 10
-9
for the next 15 Myrs (40-25 Myrs ago) to 1.0 × 10
-9
for the last 25 Myrs (Koop
et
al
., 1989). The average evolutionary rate for the hominoids (1.1 × 10
-9
) is lower
than the rates for macaque, a catarrhine (1.9 × 10
-9
) and for spider monkey, a
platyrrhine (1.8 × 10
-9
). By comparison, the line of descent from primate node to
Tarsius
shows an evolutionary rate of 3.4 × 10
-9
fixed changes site
-1
year
-1
which is
approximately half the stem-simian rate (Koop
et al
., 1989). The
hominoid slow-
down
is at its greatest in human (Li and Tanimura, 1987) although anatomically,
humans are quite divergent. Clearly, changes in certain key genes must have
assumed a critical importance. As if perhaps to emphasize this point, some gene
