Humans and the great apes
Overview of article
The question of whether humans are really related to the great apes is often at the forefront of
discussions about evolution. Creationists do not like to believe that humans and apes can be closely
related, preferring to believe that we were all created independently by a supreme being or deity.
Darwinists welcome the theory, which appears scientifically proven, that humans have evolved from
the great apes. In the following article Roger Brown presents the scientific evidence which leads to the
conclusion that humans are descended from the great apes and a sibling species to the chimpanzee.
Humans and the great apes – Roger Brown
… The problem is that a considerable breadth and depth of knowledge is needed before one has the
authority to be critical (however gifted and talented one may be). Traditionally, much of the specialist
biological knowledge required isn’t covered until A-level or first degree courses.
The main problem with understanding in biology is the enormous amount of data. As John Maynard
Smith used to say: There is ‘no single idea in biology that is hard to understand in the way in which
some ideas in physics are hard’. What is difficult in biology is the bewildering number and variety of
things that one must know about and take into account. The key ideas of evolutionary theory hold
together very coherently and are supported by an enormous range of facts. There are lots of things we
do not know about evolution, but usually they are not the things that non-biologists think we do not
know…
There is a great deal of similarity between the number and overall appearance of the chromosomes of
humans and the other living great apes (orangutan, gorilla, bonobo and chimpanzee). The five species
have a similar number of chromosomes, with the apes all having 24 pairs, and humans having 23
pairs. This might seem rather surprising, but only if you think that having the same number of
chromosomes is important evidence for common ancestry. To someone who is not familiar with basic
genetics, experimental facts and evolutionary theory this might seem like a reasonable thing to think
Briefly, the accepted scientific picture is as follows:
Comparisons of chromosomes are done by high resolution light photomicrography. Photographs of
chromosomes are cut out and arranged in standard configurations called karyotypes. When examined
closely, photographs of microscopic preparations reveal banding in the chromosomes. They show
extensive similarity in the chromosomes of the five species named above. There are about 1000
bands in the chromosomes of the great apes and the same in humans. Research has shown that
every one of these bands can be accounted for in the five species, although they are not always found
in the same place. There’s nothing new about this; the human chromosome number was determined
in the 1950s and many of the detailed similarities with great apes have been established for about 40
years.
Above: Photograph of human karyotype (male)
Most of the chromosomal differences among the five species involve inversions – sections of
chromosome that have been swapped end for end within the same chromosome. This is a relatively
common occurrence among many species, not just apes. Inversions are very important in evolution
because the genes inverted are closely linked and always inherited in a block that cannot be
separated; they are often called ‘super-genes’ and organisms possessing them often have selective
advantages.
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Another type of rearrangement is translocation (parts swapped between different chromosomes).
These are less common than inversions, but still not uncommon in organisms generally. The biggest
chromosomal rearrangement in the apes is a translocation that produces the 23 pairs found in humans
as opposed to the 24 pairs in the other great apes. … Chromosomes are numbered by convention,
starting with the largest. Chromosome two, the second largest of the human chromosomes, appears
to be formed by fusion of two medium-sized ape chromosomes. When the banding is examined, the
bands in human and ape chromosome two correspond exactly. As there is no change in the number
and nature of the bands, it looks as if there is simply a rearrangement of their positions.
Above: A simple comparison of the karyotypes of two great apes
How can this particular chromosome fusion hypothesis be tested? Well, it actually provides two
testable predictions:
1. The ends of chromosomes have a distinctive structure (called the telomere). If fusion has
occurred, evidence of this structure would be expected in the in middle of human chromosome
two.
2. A chromosome has a distinctive central part of the chromosome (called the centromere). If
fusion has occurred, there should be evidence of two centromeres in chromosome two.
This is exactly what is found. This is sound scientific reasoning and observation (unlike creationism
and ‘intelligent design’, science works by testing predictions).
So, no information was gained or lost; it was simply rearranged: human chromosome two is
homologous to two smaller ape chromosomes placed end to end. The conventional evolutionary
interpretation is that the common ancestor of the human and chimpanzee lines had 24 pairs of
chromosomes and that the change to 23 pairs probably gave rise to the hominid lineage.
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It seems to me that this scientific explanation is infinitely more plausible than the intervention of some
supernatural designer, for the simple reason that it does not require positing things for which there is
no evidence.
A common misunderstanding, often asserted, is that because such a large scale mutation (involving a
large chunk of chromosome moving from one chromosome to another) would occur in only one
individual, the animal carrying the mutation would have difficulty in finding a mate with a similar
mutation. The assumption here is that chromosomes from male and female must be similar in order to
pair up in the usual way during meiosis, if viable offspring are to result. In fact, all sorts of chromosome
contortions that enable inversions and translocations to align are known (for a range of species). I am
not sure whether it would work in this case, but, clearly, a human-chimpanzee hybridization
experiment to test this hypothesis is unethical.
However, even if alignment could not occur, the odd chromosome has to end up somewhere and its
presence need not be lethal. Elementary calculations, based on standard textbook genetics, show that
two of the permutations of offspring, one ‘ancestral normal’, the other ‘carrying the fused chromosome’
are possible and these should be viable. All the remaining four possible permutations would be lethal.
Nevertheless, two out of six isn’t bad.
If the common ancestor’s social system involved a dominant male and a ‘harem’ of females, then such
a chromosome mutation arising in the male could be rapidly distributed amongst a large number of
offspring. In this way the new chromosome pattern could become rapidly ‘fixed’ within a small isolated
‘founder’ population. This kind of social system exists in quite a number of living primates, including
some great apes, and there is no reason to believe it wasn’t thus in an ancestral ape population. Of
course, argument based on behaviour is necessarily speculative (David Attenborough and the BBC
Natural History Unit weren’t around at the time and these things don’t fossilize), but it is securely
based in ethological and evolutionary theory and the model described conforms exactly with how
speciation occurs in many other (unrelated) species.
In any case, the number of chromosomes is not important evidence of common ancestry, nor for that
matter is the fusion. The fusion accounts for a superficial difference (i.e. the number of chromosomes),
but it is the underlying genetic sequence (the genome) that exposes the relatedness of humans and
other apes.
Several quite specific genes are known that are only found in humans and the other great apes. There
are also things called ‘pseudo-genes’, which are non-functional copies of genes that result from
genetic ‘accidents’; they remain in the genome, even though they no longer function. In a sense, they
are ‘fossil genes’. The existence of the same pseudo-genes in humans and other great apes obviously
points to common ancestry. It is hard to credit that ‘intelligent design’ would replicate exactly the same
DNA errors in different species.
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It is very well established, from DNA hybridisation experiment results, that there is remarkable overall
similarity between the genomes. Analysis of a random sequence of chimpanzee DNA, compared with
a comparable sequence of human DNA shows 98 per cent similarity. Compared with a gorilla it is 97
per cent. The difference in DNA is actually far smaller than between most other different species. This
is taken to be an indication of how recently the lineages have diverged. At the gene level,
chimpanzees and humans are sibling species. The proportion of shared DNA is the most striking
evidence for the common ancestry of humans and the great apes and this interpretation is not a point
in contention by any reasonable biologist.
What seems extraordinary, of course, is that at anatomical and behavioural levels, humans and the
closest great apes differ far more than do most sibling species. This is something that many people
find hard to understand and so they may be tempted to offer non-scientific explanations.
The scientific explanation is that two per cent genome difference between humans and chimpanzees
amounts to about ten megabytes - and a lot of information can be coded in ten megabytes. There are
two types of molecular evolution: that of structural genes and that of regulatory genes. Human
structural genes are very similar to those of chimpanzees, but human regulatory genes differ. The idea
that all the major morphological differences are connected with the actions of regulatory genes is
consistent with the idea that paedomorphic development (the process whereby we have come to
resemble foetal apes) has played a major part in human evolution
Of course, part of the ten megabytes must also account for behavioural differences between humans
and chimpanzees. Although what proportion of the genome determines behavioural differences isn’t
yet known, it’s probably very small. Even in the much more closely related bonobos and chimpanzees,
where the genetic difference is just a few tenths of one per cent of their genomes, behaviour is
radically different, as zookeepers discovered when they inadvertently mixed the two species (they look
very similar): chimpanzees are amongst the most aggressive animals known, whereas bonobos are
just about the most peaceable.
Genes have clearly affected the size, shape and performance of parts of the brain quite differently in
the different ape species. Until quite recently, there was little evidence for specific brain regulatory
genes having a role in the evolutionary path between other apes and humans, but, within the last few
years, researchers have discovered a gene called PDYN that plays critical roles in regulating
perception, behaviour and memory. Analysis of the sequence structure of PDYN in humans and in
seven species of non-human primates — chimpanzees, bonobos, gorillas, orangutans, baboons, pigtailed macaques and rhesus monkeys — shows significant mutational changes in the regulatory
sequence leading to humans.
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Developmental genetics is a very active research area in labs around the world and many more
relevant discoveries about regulatory genes are likely in the near future. Within the last decade the
human genome has been determined. A draft version of the chimpanzee genome has also been
established. Within the next decade the chimpanzee genome will probably be fully determined (and
possibly that of some other great apes too). There may be another half-century of research needed
before we learn more about the genetic basis of the origin of speech (and abstract reasoning) and the
elaboration of the frontal lobes of the brain. Research should reveal a much fuller picture, but is not
really necessary to do this in order to draw general conclusions about common ancestry and human
evolution. These conclusions are pretty secure on the existing evidence.
Aspects of human culture, of course, always get in the way of popular acceptance of evolutionary
biology. Anthropocentric thinking and a number of other key ideas (mainly wrong) that have deep
religious and philosophical roots and are dominant in western thought — especially since the
Enlightenment (e.g. Locke’s tabula rasa, Cartesian dualism, Rousseau’s ‘noble savage’) — may be to
blame for many popular wrong-headed views of human nature and evolution. Certainly, the attempt to
keep humans separate from the other great apes is the (usually not too hidden) agenda of many who
attack evolution.
There are all sorts of historical examples of this from the immediate post-Darwin era. One quite
interesting one was the way Richard Owen (the eminent Victorian anatomist) claimed that the
hippocampus minor was a unique feature in the human brain, because he could not find it in the brains
of apes. In 1861 the ‘hippocampus question’ became one of the most fashionable intellectual debates
in London. It was, of course, a desperate defence against the theory of descent. Thomas Henry
Huxley proved to be a better anatomist than Owen and disproved Owen’s claims by examining the
freshly pickled brains of gorillas, then being brought back from the Congo.
When Pope John Paul II reconciled the church to evolutionary theory in 1996, he argued that there
was an ‘ontological discontinuity’ between ancestral apes and human beings. In response to this, in
his excellent book Genome, Matt Ridley suggests that the genes for the soul must lie near the middle
of human chromosome two! (Of course, the rationale for this biologist’s joke is the evolutionary
explanation of chromosome fusion, which I outlined earlier).
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Useful references
Chimpanzee Genome Project (from Wikipedia):
http://en.wikipedia.org/wiki/Chimpanzee_Genome_Project
Professor Steve Jones (Royal Society podcast):
http://www.royalsoc.ac.uk/page.asp?id=4400&tip=1
Roger Brown
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