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
other field). The controversy has now been settled (with a few dissenters) in favor of
the original molecular evidence. 
:oba:The morphological characters previously believed
to show a relation between Homo and Ramapithecus succumbed to reanalysis. The
dental arcade of Ramapithecus had been wrongly reconstructed (originally by combining
parts from different specimens). The reduced canine teeth may be because the
fossil Ramapithecus specimens were female. Martin (1985) finally removed the last
important character a thickened enamel a by reinterpreting it as an ancestral character.
Moreover, when Ramapithecus was compared with another fossil (Sivapithecus)
that was generally accepted to be a close relative of the orang-utan, and with the
orang-utan itself, it was found to show clear similarities to them. The specimens
formerly classified as Ramapithecus are now usually included in the genus Sivapithecus,
which in turn is thought to be a close relative of the ancestors of modern orang-utans
(Figure 15.25b).
In summary (simplifying things a little), molecular evidence helped to inspire a
reanalysis of the fossil evidence for human origins 
:?a with the result that a figure of
about 5 million years, and at any rate in the 4–8 million year range, is now widely
accepted for the time of origin of the hominin lineage.
pag. 460-463.
