Evolution__3rd_Edition

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Figure 15.24
The rooted angiosperm tree,
inferred by paralog rooting.
Species of the flowering plants
are written down the center.
The subtree on the left is
based on sequences of the
phytochrome A gene and the
subtree on the right is based on
sequences of the phytochrome
C gene. The two genes are
paralogs. The two subtrees are
connected by a long branch
at the bottom of the figure.
The tree is arranged in this
way, with two mirror-image
subtrees, for reasons explained
in Figure 15.23. The root is
inferred to be in the long
branch at the bottom. The
correspondence between the
subtrees for the two genes is
impressive: nodes that are the
same in the two subtrees are
indicated by capital letters.
Unlabeled nodes do not exactly
correspond in the two. The tree
was inferred by parsimony.
Slightly modified, by
permission of the publishers,
from Mathews & Donoghue
(1999).
. . . used to be linked to humans by
several characters ...
. . . implying an ancient origin for
humans
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CHAPTER 15 / The Reconstruction of Phylogeny 461
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Sagittaria
Lemna
Acorus
Chloranthus
Nelumbo
Trochodendron
Aquilegia
Lactoris
Saruma
Houttuynia
Saururus
Piper
Canella
Drimys
Calycanthus
Idiospermum
Hedycarya
Hernandia
Degeneria
Magnolia
Eupomatia
Annona
Austrobaileya
Nymphaea
Cabombacaea
Amborella
paleoanthropologists thought that Ramapithecus was a hominin: that is, it was more
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.
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