170
4 Molecular Evolution
200 species of speckled perch (Cichlidae), which
are found only in the 1-million-year-old Victoria
Lake in Africa, are monophyletic according to
their mtDNA sequences, i.e. they originate from
a single common ancestral species [273]. Molecular data have also helped to solve the problem of
the roots of the terrestrial vertebrates. Impressed
by the discovery of Latimeria chalumnae as the
surviving representative of the lobe-finned fish
(Crossopterygii), it was thought likely that here
was the origin of the tetrapods. However, the fact
that the mtDNA of Xenopus is more similar to
that of the three lungfish species than is that of
Latimeria speaks more for the Dipnoi as the forerunners of the tetrapods [272].
4.7.4 Molecular Data and the Large-Scale
Classification of Organisms
Molecular data provide evidence that all present
living organisms have a common origin, but they
do not allow unambiguous conclusions to be
drawn about the genealogical relationships of the
various groups of organisms. Protein phylogenetic trees may, in many cases, represent
authentic phylogenetic relationships between
higher eukaryotes; however, several authors
doubt that these may be expanded to include the
prokaryotes. It may well be the case that evolving
amino acid sequences come up against the limits
of difference that are set by structural-functional
restrictions and no longer represent evolutionary
distance [274]. The many attempts to evaluate the
nucleotide sequences of the 5S rRNAs in a broad
taxonomic frame are of questionable worth,
because such small rRNAs contain too little information [157]. More suitable are the rRNAs of the
small ribosomal subunit (SS-rRNAs), the sequences of which are now known completely or in part
for about 400 organisms [138, 421]. Phylogenetic
trees constructed from these data allow the three
lines of the Archebacteria, Eubacteria and Eukaryotes to be distinguished and indicate the path
from the (various) Eubacteria to the mitochondria and chloroplasts [311].
However, the attempts to use these data in the
construction of a clear branching scheme that
leads to the different groups of the eukaryotes
[65, 119] are not convincing. The evolution of the
eukaryotes appears at present not to be a tree
whose branches arise from the stem at clearly
defined intervals but a bush in which the origins
of the individual branches from the common
roots cannot be defined. The classical division of
the organisms overvalued the morphological
diversity of the multicellular organisms and
undervalued the molecular diversity of the unicellular eukaryotes and the prokaryotes. This is illustrated by the finding that three of the five algal
species assigned to the genus Chlorella are more
similar to other algae than to Chlorella vulgaris in
their SS-rRNA sequences [187]. Results continue
to accumulate which indicate that various groups
of unicellular eukaryotes had already split off
from the common evolutionary line before the
separation of the fungi, plants and animals
[19, 20, 388, 423]. There are particularly convincing arguments for a special position for the Ciliates: They have a deviant genetic code; they possess the most unusual cytochrome c of all eukaryotes; their histone H4 differs by more than 20 %
from that of all other eukaryotes, the H4 of which
differs without exception by less than 10 %; and
the other histones, the 5.8s rRNA and the tRNAs
of the Ciliates all differ markedly from those of
other eukaryotes.
4.8 Palaeo biochemistry
Quite understandably, the examination of fossils
plays only a minor role in molecular research into
animal evolution. The macromolecular biosubstances retained in fossils consist mainly of
insoluble scleroproteins which can be identified
by hydrolysis and amino acid analysis. Such substances include collagen in fossil bones and teeth
and also in invertebrates (brachiopods) since the
Palaeozoic, ceratin in turtle shells, conchiolins in
mussel shells, and other proteins in the remains of
extinct graptoliths, in the calcareous bodies of the
foraminifers and in the silicic acid skeletons of the
radiolarians. The egg shells of dinosaurs contain
proteins with a high glycine content and which are
similar to the calcium-binding proteins of bird
eggs [220]. The amino acids of fossil proteins are,
however, subject to change through spontaneous
chemical reactions; thus, L-aspartate is converted
to the D-isomer at the rate of 0.1-0.25 % per
year; D-allo-leucine is formed from L-Ieucine at
the much slower rate of 0.001 % per thousand
years [84]. In spite of these age-related chemical
changes, proteins extracted from 60- to 80million-year-old mussel shells still show immunological cross-reactivity with the corresponding
proteins of present-day mussels [431]. Unfortunately, little phylogenetically useful information
4 Molecular Evolution
200 species of speckled perch (Cichlidae), which
are found only in the 1-million-year-old Victoria
Lake in Africa, are monophyletic according to
their mtDNA sequences, i.e. they originate from
a single common ancestral species [273]. Molecular data have also helped to solve the problem of
the roots of the terrestrial vertebrates. Impressed
by the discovery of Latimeria chalumnae as the
surviving representative of the lobe-finned fish
(Crossopterygii), it was thought likely that here
was the origin of the tetrapods. However, the fact
that the mtDNA of Xenopus is more similar to
that of the three lungfish species than is that of
Latimeria speaks more for the Dipnoi as the forerunners of the tetrapods [272].
4.7.4 Molecular Data and the Large-Scale
Classification of Organisms
Molecular data provide evidence that all present
living organisms have a common origin, but they
do not allow unambiguous conclusions to be
drawn about the genealogical relationships of the
various groups of organisms. Protein phylogenetic trees may, in many cases, represent
authentic phylogenetic relationships between
higher eukaryotes; however, several authors
doubt that these may be expanded to include the
prokaryotes. It may well be the case that evolving
amino acid sequences come up against the limits
of difference that are set by structural-functional
restrictions and no longer represent evolutionary
distance [274]. The many attempts to evaluate the
nucleotide sequences of the 5S rRNAs in a broad
taxonomic frame are of questionable worth,
because such small rRNAs contain too little information [157]. More suitable are the rRNAs of the
small ribosomal subunit (SS-rRNAs), the sequences of which are now known completely or in part
for about 400 organisms [138, 421]. Phylogenetic
trees constructed from these data allow the three
lines of the Archebacteria, Eubacteria and Eukaryotes to be distinguished and indicate the path
from the (various) Eubacteria to the mitochondria and chloroplasts [311].
However, the attempts to use these data in the
construction of a clear branching scheme that
leads to the different groups of the eukaryotes
[65, 119] are not convincing. The evolution of the
eukaryotes appears at present not to be a tree
whose branches arise from the stem at clearly
defined intervals but a bush in which the origins
of the individual branches from the common
roots cannot be defined. The classical division of
the organisms overvalued the morphological
diversity of the multicellular organisms and
undervalued the molecular diversity of the unicellular eukaryotes and the prokaryotes. This is illustrated by the finding that three of the five algal
species assigned to the genus Chlorella are more
similar to other algae than to Chlorella vulgaris in
their SS-rRNA sequences [187]. Results continue
to accumulate which indicate that various groups
of unicellular eukaryotes had already split off
from the common evolutionary line before the
separation of the fungi, plants and animals
[19, 20, 388, 423]. There are particularly convincing arguments for a special position for the Ciliates: They have a deviant genetic code; they possess the most unusual cytochrome c of all eukaryotes; their histone H4 differs by more than 20 %
from that of all other eukaryotes, the H4 of which
differs without exception by less than 10 %; and
the other histones, the 5.8s rRNA and the tRNAs
of the Ciliates all differ markedly from those of
other eukaryotes.
4.8 Palaeo biochemistry
Quite understandably, the examination of fossils
plays only a minor role in molecular research into
animal evolution. The macromolecular biosubstances retained in fossils consist mainly of
insoluble scleroproteins which can be identified
by hydrolysis and amino acid analysis. Such substances include collagen in fossil bones and teeth
and also in invertebrates (brachiopods) since the
Palaeozoic, ceratin in turtle shells, conchiolins in
mussel shells, and other proteins in the remains of
extinct graptoliths, in the calcareous bodies of the
foraminifers and in the silicic acid skeletons of the
radiolarians. The egg shells of dinosaurs contain
proteins with a high glycine content and which are
similar to the calcium-binding proteins of bird
eggs [220]. The amino acids of fossil proteins are,
however, subject to change through spontaneous
chemical reactions; thus, L-aspartate is converted
to the D-isomer at the rate of 0.1-0.25 % per
year; D-allo-leucine is formed from L-Ieucine at
the much slower rate of 0.001 % per thousand
years [84]. In spite of these age-related chemical
changes, proteins extracted from 60- to 80million-year-old mussel shells still show immunological cross-reactivity with the corresponding
proteins of present-day mussels [431]. Unfortunately, little phylogenetically useful information