Contingency : Replaying life's tape
Contingency is the concept that each step in evolution depends on all
the steps before, and that if you repeated the history of evolution on
planet Earth then it would take a different path because some of
those steps would be different. Although it is a somewhat
philosophical concept, it is worth thinking about.
The late biologist Stephen Jay Gould discusses the concept of contingency at length throughout his book Wonderful Life and concludes that life today would be very different had other creatures dominated in the Cambrian period (Ref 1.) In support of this view a recently announced experiment on bacteria investigated the consequences of contingency, with some very interesting results (Ref 2.)
The experiment has been running for 20 years and involves taking 12 identical populations of bacteria and watching them over 40,000 generations. The particularly interesting feature of this experiment is that the scientists froze samples ever 500 generations so they could go back and replay from ancestral populations. This allows them to narrow down which population any particular mutation appeared in, and to track its progress through the generations.
After around 31,500 generations a mutation allowed one of the populations to make use of new food source, citrate. Scientists traced the change back to at least two mutations, one of which had happened without any discernible effect in the ancestral population. The interesting part is that when they replayed from that ancestral population, the subsequent evolution of the new trait was not guaranteed. In other words it wasn't certain that the path to the second mutation would be taken even within a controlled environment.
Gould's speculation would appear to be confirmed by the experiment, the question remains as to whether or not contingency is either surprising or interesting. In the Crucible of Creation Simon Conway Morris counters that although the precise creatures that make up life on the planet would be different, the same environmental niches would exist and similar creatures would evolve to exploit them (Ref 3) so contingency isn't as crucial as Gould considered it. The two scientists went as a far as to have a written "discussion" on their different viewpoints (Ref 4.)
Although it is true that life will expand into available niches, it is also intuitive that some events must have been so major as to have changed life's course completely. The meteorite which appears to have brought about the extinction of the dinosaurs appears at first glance to be one such event but it can be argued that life would still exist on Earth had that meteorite not struck and perhaps intelligent life would still have arisen e.g. from creatures such as Troodonts.
A much more fundamental, though controversial event is the Snowball Earth hypothesis. The idea is that the whole earth has periodically cooled down to the extent that the majority of the planet became frozen. This would have had a dramatic effect on life on the planet as it may only have been possible to survive in isolated communities at undersea vents, where access to sunlight would have been restricted.
The theory may also help answer the enduring mystery as to why life finally evolved multi-cellular forms after two billion years of single celled organisms. The hardened conditions may have given the impetus needed for life to evolve in the isolated communities close to the vents. The eventual warming of the planet (due to the greenhouse effect from volcanic CO2) would finally allow the long separated populations to compete for the newly released resources. Were the isolated harsh environments and eventual competition the spurs needed to evolve multi-cellular life?
The period for when the Earth is hypothesised to have been frozen is from 790 to 630mya which is roughly 100 million years before the Cambrian period and just before the Ediacaran period which is when the first multi-cellular life forms start appearing in the fossil record (Ref 5.)
Was the Snowball Earth event responsible for knocking life out of the comfortable single celled niche it had occupied for two billion years? We may never know for sure, but it is possible that if the Earth hadn't frozen then bacteria would have remained the most advanced life-forms on the planet!