EVOLUTION IN REVERSE
Animals can evolve back towards their ancestors. Scientists used to think this was impossible, but new algorithms and gene analyses demonstrate that this has happened to lizards, frogs, and even humans.
Humans grow tails, walking sticks fly again, and a frog that found its teeth after losing them 225 million years ago. It seems that evolution isn’t a one-way street.
High up in the Andes, a heavily pregnant lizard is crawling around the rocks. Suddenly, she stops and spreads her hind legs. Shortly afterwards a small head protrudes, wrapped in a thin, transparent membrane, which the mother immediately tears to pieces. Fifteen minutes later, three babies are moving about.
This is how the small lizard and its peers have had offspring since their ancestors took a dramatic evolutionary step 30 million years ago and began to give birth to living offspring instead of laying eggs. The change in the lizards’ biology was so significant that scientists used to consider it impossible that they might go back to laying eggs again. However, a new study carried out in 2018 by scientists from Chile and Australia has revealed that it has already happened to some lizard species.
This discovery and other new studies are demonstrating that animals can regain features that their ancestors lost 100 million years ago or more. Evolution may jump back and forth much more than we used to imagine, and the phenomenon can even be observed in our own bodies.
Gone for good, says Dollo’s Law
Major evolutionary changes such as the South American lizard’s shift from eggs to live offspring are quite normal in nature. Snakes lost their legs, humans said goodbye to their tail, and penguins can no longer fly. New conditions can make features redundant or inconvenient, so that animals sometimes evolve to get rid of them.
And according to Dollo’s Law, once they are gone, they are gone for good. The law, introduced by the Belgian palaeontologist Louis Dollo in 1890, stated that evolution cannot be reversed.
Since then, the idea has dominated the theory of evolution – and with good reason. The recreation of a lost feature would seem to require a very unlikely process in which a series of altered genes must change back into their former versions. In the case of the lizards, the genetic changes would be considerable. The change from laying eggs to giving birth to live offspring requires birth canal restructuring, a much thinner egg shell, development of a placenta, and immune system changes so that the mother does not reject the baby as a foreign body.
Dollo’s Law has held sway for more than 100 years, and not only because it seems sensible in principle. So far, scientists have simply not had the tools to disprove it. But sophisticated algorithms and new DNA reading methods are making it possible to get a more detailed picture of millions of years of evolution – and so reveal some of evolution’s biggest secrets.
Algorithms turn back time
Family trees are the key to understanding the pasts of animals, as of ourselves. A family tree can tell us that our closest and most remote mammal ancestors are furred. So we must descend from furred animals.
Similarly, Australian and Chilean scientists revealed for the first time in 2018 that several groups of egg-laying lizards from South America descend from ancestors that gave birth to live offspring. So in the course of their evolution the South American lizards have gone from eggs to live offspring and back to eggs again. The surprising
conclusion is based on their family tree, where the egg-laying lizards are surrounded by lizards that give birth to live offspring.
The method seems simple, but until recently such a study would not have been possible. In order to draw up a family tree, scientists first need to form a general view of all the ways in which the species of the family tree can be related. Four species add up to 15 possible versions of the family tree, and then the scientists need to calculate which one is the most likely based on the animals’ genes and other factors. If there are 10 species, they can be related in 34 million different ways, and it becomes much more difficult to find the most likely family tree. The study of the South American lizard includes the genes of 258 species.
This ‘mapping out’ of DNA from so many species would have been almost impossible only a few years ago, but today it is possible thanks to new and relatively cheap DNA sequencing methods. Even then the analysis of the species’ kinship requires so much computing power that scientists have only recently been able to churn all the data. The scientists’ algorithms sorted through a total of 500 possible family trees – a process that might take weeks – to reach the most likely family tree.
Apart from the genes from the lizards, the scientists also fed their algorithms data about extinct lizard fossils and the environments of individual species. From this the scientists could reveal that the first change from egg to live offspring probably took place when the Andes mountains rose up 30 million years ago. The new mountains lifted the lizards so high that the air around them became colder. Cold is lethal for embryos that develop in eggs outside the mother’s body, so the lizards began to keep their embryos in their stomachs until the embryonic stage had been completed. The egg shell was replaced by a thin membrane, and the offspring was viable at birth. Later, some lizards moved into the warmer lowlands at the base of the mountains, making it warm enough to lay eggs again.
The lizards are just the most recent example of animals that violate Dollo’s Law. Improved technology has allowed scientists to find others. A study from 2011 showed that just one of the world’s 6000+ frog species had developed teeth in its lower jaw – a feature that the ancestor of frogs lost at least 225 million years ago.
Humans also seem to have violated the law. Two muscles at the top of our backs merged into one for our remote mouse-like ancestors 89 million years ago. But a few million years ago, evolution did a back flip, and the muscle again divided in two.
Most genes survive the loss
All these violations of Dollo’s Law have forced scientists to reconsider the function of evolution. Instead of imagining that the genes of a specific characteristic are lost when it disappears, scientists now think that in many cases the genes remain intact but simply inactive. And as long as the genes are intact, the lost feature can be recreated.
Scientists have found an example of this phenomenon in poultry. The birds lost their teeth more than 60 million years ago, but most of the genes that can produce teeth are still fully functional. By changing only one single gene, scientists can activate all these genes and make poultry develop teeth again. The birds’ tooth genes probably survived