Evolution__3rd_Edition

462 PART 4 / Evolution and Diversity
(a) (b)
Sivapithecus
Orang-utan Gorilla Chimp Ramapithecus Homo Orang-utan (incl. “Ramapithecus” ) Gorilla Chimp Homo
>9–12 million
years ago
Figure 15.25
Relations of Homo, other great apes, and Ramapithecus,
according to (a) original paleontological and morphological
Molecular evidence suggested a
more recent origin for humans
>9–12 million
years ago
About 5 million
years ago
evidence, and (b) molecular (and revised paleontological and
morphological) evidence. (The dashed lines imply uncertainty
in the order of the human–chimpanzee–gorilla split.)
In the early 1960s, Goodman (1963) first demonstrated the molecular similarity of
humans and other great apes; but the molecular argument for a recent human–ape split
was most influentially made in a paper by Sarich & Wilson (1967). Sarich and Wilson
used an immunological distance measure. The method is similar in philosophy to DNA
hybridization, but differs in the exact molecule used.
To measure immunological distance, Sarich and Wilson first made an antiserum
against human albumin by injecting human albumin into rabbits (albumin is a common
protein which circulates in the blood). They then measured how much that antiserum
cross-reacted with the albumin of other species, such as chimps, gorillas, and
gibbons. The antiserum recognizes the albumins of closely related species, because they
are similar to human albumin; but it does not recognize them quite as efficiently as it
does human albumin. The degree of cross-reactivity gives a measure of the immunological
distance (ID) between a pair of species. ID increases among phylogenetically
more distant relatives, and the relative rate test (Box 7.2, p. 166) suggests that ID
increases at a constant rate through time; immunological distance is a sort of molecular
clock. The clock can be calibrated using the fossil record for some of the studied species,
and the ID can then be used to estimate the divergence time for other pairs of species.
The results of this method suggest that Homo and the other great apes have too short
an ID to fit with a pre-Ramapithecus divergence: Sarich and Wilson suggested humans
and chimps diverged only about 5 million years ago. Subsequent molecular work has
supported them. The DNA hybridization results that we looked at earlier suggest a similar,
if perhaps slightly older, figure (see Figure 15.12), and other molecules suggest a
figure of 3.75–4 million years. The corollary is that if Homo diverged from chimps and
gorillas 5 million years ago, it cannot be more closely related to Ramapithecus than to
the living great apes. The phylogeny must be more like Figure 15.25b.
So the molecular and fossil evidence disagreed. A controversy began, in which both
the molecular and morphological evidence was challenged (often by experts in the
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