There’s been a long debate regarding whether evolution can be reversed or not. The general trend has been that it can not. The idea goes that once one evolutionary pathway has been crossed, it cannot be retraced back to its origins. It turns out that is not entirely true.
Says [researcher] Henrique, ‘In 2001 we showed that evolution is reversible in as far as phenotypes are concerned, but even then, only to a point. Indeed, not all the characteristics evolved back to the ancestral state. Furthermore, some characteristics reverse-evolved rapidly, while others took longer. Reverse evolution seems to stop when the populations of flies achieve adaptation to the ancestral environment, which may not coincide with the ancestral state.
What the researchers did was subject fruit flies to various selection pressure for multiple decades, i.e., they changed their environment over and over. The ‘end’ result was fruit flies that were markedly different in their traits as compared to the original specimens. That’s evolution. Children should understand that. What happened next was the researchers mimicked the original environment of the fruit flies from decades gone by. In response, the fruit flies adapted to those environments, possessing many of the same allele frequencies they originally had. What I find particularly interesting is that they did not evolve exactly the same, but they still evolved in a way that was similar to the original phenotypes. This helps to explain why sharks and horseshoe crabs remain so similar for so long: the gene pool of the population centers around certain allele frequencies because, well, they work. Change may happen – in fact, it certainly does – but ancestral pheno- and genotypes can evolve to such similar future counterparts as to make little difference in show, even though we know there to actually be differences, at least in contigency. It’s a bit like how two people of very different backgrounds and even different alleles can come to have the similar tones to their skin. Their evolutionary contigency, or histories, are different, but the result is virtually the same.
Another point of note here is that evolution can produce similar things, but it will almost never produce the exact same thing. The history of life, if rerun, would be much, much different in all likelihood. When exolife is discovered, we’ll have indirect confirmation of this. Until then, it should be important for people to realize that nothing in biology is inevitable – including humans.