Archaea
are single-celled organisms, 3 billion years old, and older than bacteria.
Humans share around 200 archaea genes that are common to all living things
(e.g. respiration, utilizing food), including one on chromosome 6 that tells other
genes what to do.
For the
1st billion years only single-celled organisms (archaea and
bacteria). Then around 2 billion years ago, more complex organisms appeared; the
consensus now is that this occurred when an archaea enveloped a bacteria. In
the plants and animals that developed, the bacteria function as a cell
powerhouse – the mitochondria.
Amphioxus
(lancelets) are invertebrates that split from vertebrates more than 520 million
years ago; their genomes hold clues about evolution, particularly how
vertebrates have employed old genes for new functions. Amphioxus have a dorsal
nerve cord surrounded by a cylinder of cells – the notochord.
Vertebrates
appear some 450 million years ago; now thought that their genome is essentially
quadrupled amphioxus genome. Analysis of the human genome shows quadrupled
‘old’ genome, though not all genes are there 4 times (some 3 times, some twice
and some only once).
A boa
constrictor has 304 vertebrae while humans have 33. The difference is not in
the genes but how they are expressed. One gene tells a developing organism to
grow a vertebra; another gene acts as a clock counting how many times this
happens and how fast, resulting in the different numbers. An examination of
mammal skulls reveals that they are all made of the same set of bones, though
the shape varies due to when relevant genes were switched on and for how long
in the embryo stage.
In the
reproductive process, chromosomes divide and exchange genetic material in
meiosis. This process can result in ‘misspellings’, duplications, inversions
and loss of specific genes. Successful changes determine evolution. There are
specific places where this happens, known as recombination hotspots and marked
by specific 13 base pair sequences. Each species has its own recombination
hotspot locations. Ch.5 has a gene that controls where hotspots occur. Changes
here make organisms incapable of cross-breeding, leading to a split in a
population and ultimately two species.
In human
colour vision, the gene for seeing blue is on ch.7 while those for seeing red
and green are on the X ch. The red and green genes are very close together and
98% similar; thought that the original one was duplicated and then took on a
new function. Other species can see infra-red and ultraviolet. [Now thought that some women can distinguish
more shades of green, since they have a version of the green vision gene from
both parents. In females, in each cell one of the X ch is switched off in a
random fashion, so some cells use the maternal X and others the paternal. This
explains how women are carriers for conditions they do not suffer from.]
‘Fossil’
genes are present but no longer functional due to random changes (e.g. few
people can now smell androstenone, a steroid found in sweat and urine).
Humans
are very similar to chimpanzees but have weaker muscles, less developed sense
of smell and far fewer digestives enzymes: humans can no longer eat a diet of
only raw food – we have replaced our lack of enzymes by cooking.
There is
very little difference between humans, who are a very inbred population,
suggesting that we were a rare species for most of our history and descended
from a tiny original population. DNA analysis can trace maternal line origins.
Early modern humans left Africa in two groups; group 1 went east to south India
and that lineage is only found there while group 2 went to Europe and thence to
north India.
The
Indian caste system has restricted breeding populations over 108 generations
(3,000 years) resulting in medical problems and genetic disorders with varying
degrees of severity. Members of the merchant class (Vaishyas) cannot break down
a commonly used anaesthetic and remain unconscious for long periods so a
different anaesthetic must be used.
Some of
the genome has unidentified functions; it was initially thought that this was
irrelevant and known as ‘junk code’. Now known that these stretches are often
found across species and it is now thought that these areas tell genes to
switch on and off, especially in embryo development.
Many
traits (e.g. height) and specific conditions are of polygenetic origin.
BBC TV documentary 'The Gene Code' presented by Dr Adam Rutherford; broadcast April 2011.
