New York, June 21 :
How much does our evolution depend on chance mutations? Quite a lot, say scientists.
A team of evolutionary biologists has resurrected an ancient protein ancestor of an important human protein as it existed hundreds of millions of years ago and then generated a huge number of alternative histories that could have ensued.
Tracing these alternative evolutionary paths, the researchers discovered that the protein – the cellular receptor for the stress hormone cortisol – could not have evolved its modern-day function unless two extremely unlikely mutations happened to evolve first.
These “permissive” mutations had no effect on the protein’s function, but without them the protein could not tolerate the later mutations that caused it to evolve its sensitivity to cortisol.
In screening thousands of alternative histories, the researchers found no alternative permissive mutations that could have allowed the protein’s modern-day form to evolve.
“This very important protein exists only because of a twist of fate,” said Joe Thornton from University of Chicago in the US.
“If our results are general – and we think they probably are – then many of our body’s systems work as they do because of very unlikely chance events that happened in our deep evolutionary past,” he added.
The researchers focused on the glucocorticoid receptor (GR), a key protein in the endocrine system that regulates development and stress responses in response to the hormone cortisol.
They resurrected the gene for ancestral GR as it existed around 450 million years ago, before it evolved its capacity to specifically recognise cortisol.
They included a handful of mutations that occurred slightly later that allowed the protein to evolve its cortisol recognition.
The researchers tested many thousands of variants but found none that restored the function of GR other than the historical mutations that occurred in actuality.
The findings appeared online in the journal Nature.