Homo-Evidência Molecular muda Datação Radiométrica
Enviado: 01 Mar 2011, 23:12
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Homo-Evidência Molecular muda Datação Radiométrica
15.13 Molecular evidence successfully challenged
paleontological evidence in the analysis of human
phylogenetic relations
It is always interesting when two independent lines of evidence, from very different
fields, are applied to the same question. This sections looks at a conflict between fossil
and molecular evidence concerning the time of origin of the human evolutionary
lineage.
Mark Ridley-Evolution-3º ed.
“Ramapithecus” (which is now classified in the genus Sivapithecus) is a group of
fossil apes that lived about 9–12 million years ago. Until the late 1960s, almost all
closely related to Homo than to chimpanzees and gorillas (Figure 15.25a). (Hominoids
(formally superfamily Hominoidea) are the group of all great apes, including humans;
hominins (formally subfamily Homininae) are the narrower group of Homo and the
australopithecines.) Ramapithecus and Homo apparently shared a number of derived
characters. For example, Homo has a rounded, “parabolic” dental arcade, whereas
chimps have a more pointed dental arcade. The dental arcade of Ramapithecus was initially
thought to be shaped more like Homo. Secondly, Ramapithecus’s canine teeth
were thought to be relatively diminished compared with its other teeth, as in Homo but
unlike chimps (in which the canines, especially in males, are large). Thirdly, Homo and
Ramapithecus were thought to share, as a derived condition, a thickened layer of tooth
enamel, unlike the thinner layer in other apes (and which was thought to be the condition
in the ancestors of the Homininae).
This morphological and paleontological argument for a relation between Homo
and Ramapithecus has a classic form: a set of character states are shown to be shared
uniquely by these two species, and the characters are derived within the larger group
of Hominoidea. The corollary was that the human lineage must have split from the
great apes at least 12 million years ago, because Ramapithecus is nearer to us than to the great apes.
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
other field). The controversy has now been settled (with a few dissenters) in favor of
the original molecular evidence. 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.
viewtopic.php?f=1&t=15824
Homo-Evidência Molecular muda Datação Radiométrica
15.13 Molecular evidence successfully challenged
paleontological evidence in the analysis of human
phylogenetic relations
It is always interesting when two independent lines of evidence, from very different
fields, are applied to the same question. This sections looks at a conflict between fossil
and molecular evidence concerning the time of origin of the human evolutionary
lineage.
Mark Ridley-Evolution-3º ed.
“Ramapithecus” (which is now classified in the genus Sivapithecus) is a group of
fossil apes that lived about 9–12 million years ago. Until the late 1960s, almost all
closely related to Homo than to chimpanzees and gorillas (Figure 15.25a). (Hominoids
(formally superfamily Hominoidea) are the group of all great apes, including humans;
hominins (formally subfamily Homininae) are the narrower group of Homo and the
australopithecines.) Ramapithecus and Homo apparently shared a number of derived
characters. For example, Homo has a rounded, “parabolic” dental arcade, whereas
chimps have a more pointed dental arcade. The dental arcade of Ramapithecus was initially
thought to be shaped more like Homo. Secondly, Ramapithecus’s canine teeth
were thought to be relatively diminished compared with its other teeth, as in Homo but
unlike chimps (in which the canines, especially in males, are large). Thirdly, Homo and
Ramapithecus were thought to share, as a derived condition, a thickened layer of tooth
enamel, unlike the thinner layer in other apes (and which was thought to be the condition
in the ancestors of the Homininae).
This morphological and paleontological argument for a relation between Homo
and Ramapithecus has a classic form: a set of character states are shown to be shared
uniquely by these two species, and the characters are derived within the larger group
of Hominoidea. The corollary was that the human lineage must have split from the
great apes at least 12 million years ago, because Ramapithecus is nearer to us than to the great apes.
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
other field). The controversy has now been settled (with a few dissenters) in favor of
the original molecular evidence. 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.
