Frog finds its lost teeth after 225 million years
Frogs lost their lower teeth 330-230 million years ago. Five million years ago, one species got them back again.
Neurotransmitters trigger the formation of teeth
During a frog’s embryonic development, a series of mouth genes are activated resulting in different neurotransmitters causing a layer of cells to produce a tooth bud. In the vast majority of frog species, the genes of the lower part of the mouth are not activated.
Genes are active all over the mouth
In one single frog species, the South American Gastrotheca guentheri, the tooth genes are apparently activated in both the lower and the upper parts of the mouth, and neurotransmitters make sure that tooth formation is triggered in both places.
Frog ends up with ancient set of teeth
The Gastrotheca guentheri frog grows a fully developed set of teeth in both the lower and the upper parts of its mouth. The formation of the lower teeth probably happens in the same way as it once did in a remote frog ancestor at least 230 million years ago.
in spite of the loss of teeth because the genes also have other useful functions in the animals’ bodies. The vast majority of genes are involved in more than one process, and they can play roles in several types of tissue. The birds’ tooth genes may continue to play an important role, in feathers and elsewhere, even though they are no longer active in the mouth.
Easy change to birth
Changes of gene activity rather than changes to the genes themselves also seem to be key to the lizards’ journey from egg-laying to live births and back again.
An international team of scientists in 2019 studied two closely-related Chinese lizard species, one that lay eggs and one that gives birth to live offspring. The scientists identified the genes that were responsible in each case and studied them. Much to their surprise, the results indicated that a change from egg-laying to live offspring is not very complicated in genetic terms.Again the difference between the two species were primarily differences in gene activity. The genes responsible for the egg shell also exist in lizards which give birth to live offspring, but they find much less expression. And since the genes are still present, the steps back to egg-laying are not as extreme as previously believed. That conclusion is supported further by the same reverse from live birth to egg-laying being observed in some snakes. Most boas have abandoned egg-laying and now give birth to living offspring, but the Eryx jayakari desert snake’s nutrient-deficient surroundings have led the animal back to laying eggs, according to a major study by the American Yale University, in which scientists mapped out a family tree including 41 boa species.
An Australian reptile species provides us with an even more direct impression of the transition between egg-laying and live births. The Saiphos equalis lizard makes use of either strategy, depending on its habitat. Near the coast, the lizard lays eggs, but in the mountains it gives birth to live offspring. The fact that one single species can exhibit such different behaviours supports the theory that evolution can take huge steps without dramatic gene change.
Ancient traits appear in embryos
Animals and humans keep the genetic tools for long-forgotten jobs or developing longlost body parts, and this is particularly clear during their embryonic development. Whales and dolphins both descend from a four-legged terrestrial animal, and they still develop small hind legs when the embryo is in a very early stage. In the vast majority of cases the limbs disappear completely before birth, but exceptions exist.
In humans, a tail can be observed in the sixth week of embryonic development. It usually disappears, but again, exceptions exist. There are several dozen examples of children that have been born with tails, and scientists still discuss exactly how and why the tails are formed. An even more ancient trait can be observed in human embryos: gills, in spite of our ancestors giving up gills 350+ million years ago. The retention of such ancient traits allows animals to hold on to possibilities for ‘reverse’ evolution.
Return of the dinosaurs
In recent years, scientists have tried to find out exactly what is required to rewind animal evolution. Some scientists would like to be able to recreate long extinct species such as mammoths or dinosaurs, and several of the changes might prove so simple that they can happen naturally without our help – as in the case of the egg-laying lizards and the frog with teeth in its lower jaw.
Birds evolved from their dinosaur ancestors, and they changed a lot in the process. For one thing they lost their teeth, yet one small adjustment of the activity of two genes could give them back their teeth. Scientists know a natural mutation that can make this adjustment take place, but the mutation is lethal, so the embryos never hatch. In the future, a more harmless mutation might result in an adult bird with teeth. In the lab, scientists have also discovered small adjustments that could give birds leg-bones and skulls reminiscent of the ones on related dinosaurs such as the Velociraptor of the Cretaceous – so the birds have several opportunities to take a step or two back towards their remote ancestors.
The South American hoatzin bird probably already took such a step. Most birds have no claws or only small claws on their wings, but in the hoatzin’s offspring the claws look like those on the bird’s ancestors. Scientists now think that earlier in its evolution the hoatzin lost the use of its claws, only to regain them later because the offspring found them useful for climbing trees.