Humans underwent rapid evolution about 5 million years ago that
dramatically differentiated us from chimps by enhancing the intra-cellular structures in the brain to allow for greater intelligence/adaptability.
It was discovered that (A) or thymine (T) changed consistently to cytosine (C) or guanine (G) over relatively small sections of non-coding DNA.
This is called a weak-to-strong bias. The regions which these events occurred
are referred to as HAR's (Human accelerated regions).
As yet, no explanation has been offered for these substitutions.
Looking at HAR1, we can see that there is only 1 substitution
between an ape and a chicken but 18 between an ape and a human.
"The HAR1 region showed the greatest variance in humans. It is a 118 base pair DNA
segment with 18 bases that differ from the same region in the other species. From
an evolutionary point of view less than one (0.27) base change(s) is expected in
the HAR1 DNA sequence in the time elapsed since chimps and humans diverged from their
common ancestor. Eighteen changes in human HAR1 equates to 67 times the number of changes expected."
"HAR1 extends over 1.2 kb,
a region far larger than HAR1 itself. Such changes, which also appear to
characterize the HARs as a group, undoubtedly serve to strengthen RNA helices
against dissociation and may promote enhanced expression or stability.
Nevertheless, the weakto-strong bias in HAR1 nucleotide substitutions remains
"By definition, there’s enough time because the evolutionary rate is calculated
by determining the number of changes between the common ancestor of humans and
chimps and then dividing that number by 6 million years—the amount of time that
has elapsed since the divergence of humans and chimps. To put it another way,
“the deck is rigged.” So, of course, there’s enough time (from the evolutionary
perspective) for humans to have evolved from chimps. Or is there? Perhaps not always,
even when using a rigged deck.
Additionally, the equivalent of HAR1 in all those other species is very highly conserved.
The researchers report that “only two bases (out of 118) are changed between chimpanzee
and chicken.” There are some attempts in the scientific literature to explain the phenomena
of “accelerated evolution.” However, HAR1 doesn’t fit into the explanations being offered2
because it is so highly conserved.3 In addition to the astonishing number of changes in human
HAR1, the changes reflect GC-biased conversion, another unexpected phenomena in which
adenine (A) or thymine (T) change consistently to cytosine (C) or guanine (G)."
Further reading :
To me, this is one of the most interesting areas in modern day biology as
it not only underlines the sheer complexity of the human brain and it's
susceptibility to even the smallest changes in DNA but shows that our
evolution is not linear by any means.
The most intriguing question to come from this is how did it happen?
What selective forces came into play to cause this accelerated evolution
and how were so many changes made on such highly conserved regions