Switching between hiking gear and lab coat, scientists probe the evolution of dinosaurs
An evolutionary biologist has begun editing genes in living fish eggs to probe how fins became hands and feet
Anchiornis and Tinamou
Evolutionary biologists are making odd creatures in their labs by tweaking eggs and embryos. In one instance: a bird with a snout. They are doing this to scrutinise big events in evolution, such as how dinosaurs evolved to birds and fish to land animals. The experiments are run by a new breed of researchers who are equally at home hiking for fossils as they are editing DNA.
Neil Shubin, a professor at the University of Chicago, carried his rock hammer into dry valleys of Antarctica last winter to dig for fossil gold – so-called “missing links” such limbed fish. His fossils will be shipped back this April; meanwhile, Shubin has begun editing genes in living fish eggs. His goal: to rewind evolution to intermediate anatomies and probe how fins became hands and feet.
Shubin is a celebrity in evolutionary biologist circles – he discovered a widely reported missing link in the Canadian Arctic, a 375-year-old fossil “fishapod” (Tiktaalik) with a neck, arm and wrist. This winter, he reckons his team uncovered promising sections from the evolutionary tree, from around 380 million years ago. This is around the time fish moved to land and some of his new fossils have anatomies between fish fins and limbs.
“Such fossils can show us the sequence by which fins were transformed into limbs,” says Shubin. “But it doesn’t tell us how this major transformation happened at the level of the genes.”
He has used a revolutionary gene-editing technique, called Crispr, to turn off master genes in fish fins, creating abnormally small-finned fish. Last year he shed light on the mystery of which fish bones gave rise to our fingers.
Shubin is not alone in switching between hiking gear and lab coat. Scientists recently changed the embryo of a bird so it grew a snout, which was dubbed the “dino-chicken”. Its creator was Bhart-Anjan Bhullar, professor of geology at Yale University in Connecticut. He is fascinated about the origins of birds and has spent long hours studying their fossils.
This led him to manipulate chickens to replicate a different time points on their route from reptilian ancestors. Just a minor tweak gave them a snout. “We replicated the molecular signalling in the ancestor. By doing that, we resurrected a condition that hasn’t existed in the bird lineage in the natural world in over 65 million years. It was very satisfying because it showed a causal connection between genetics and actual evolutionary change.”
He targeted a group of master genes in bird embryos, blocking the proteins they made. “It turns out that the beak of a bird is really an overgrown version of a bone that is very tiny in most vertebrate heads. For us it just holds our incisors in place,” says Bhullar.
The unusual creatures were not allowed to hatch, though Bhullar believes they would have survived just fine as snouted chicks. The fact you can get dinosaur anatomy into a living bird isn’t a huge surprise, says Bhullar, since birds are small dinosaurs.
In his Chicago lab, when Shubin edited the DNA of fish, he was shocked to discover the bones in our hands appear to have evolved from fish fin rays, a different sort of bone. He is now using Crispr gene editing to generate small-finned fish mutants to recapture the early anatomy of the first “fishapod” that flopped to dry land.
In the University of California, San Diego, evolutionary biologist Prof Kimberly Cooper is using Crispr to rewind evolution too. She is investigating an odd-looking species called the jerboa, a three-toed desert rodent with greatly elongated back legs, fused leg bones and no muscles around its feet. It is closely related to a mouse, but shapes up like a miniature kangaroo. Her lab is placing key genes from the jerboa into a mouse to create a mouse with jumping legs.
“Crispr allows us take genes from the jerboa and put them into a mouse to give them jerboa-like traits,” says Cooper. “We may get a mouse that has three toes, not five, or a mouse with five small toes. But that will tell us about the gene.”
Crispr will increasingly be used to study evolution. “Evolutionary developmental biology proposes that changes during the development of an organism can have profound importance in the evolution of species,” explains Dr Rainer Melzer, a geneticist in University College Dublin who studies flower evolution. “We want to be able to mutate genes in an organism in a targeted way and see what happens and Crispr greatly facilitates that.” He too plans to use Crispr in his research.
THE DINOSAURS ARE WITH US
Birds are small dinosaurs, says biologist Bhart-Anjan Bhullar at Yale. He’s already created a bird with a snout by making a small change in a chicken embryo, essentially turning back the hand of time.
The big round head, bulbous eyes and short face of living birds is “cartoon character cute syndrome”, and the explanation is simple – from puppies to fish, this is how all vertebrates heads look as babies and hints at where bird came from. “Essentially they are little baby versions of dinosaurs that never grow up,” explains Bhullar.
“Birds stop growing earlier [than dinosaurs] and look like their babies,” says Bhullar. One of his studies in the journal Nature showed that Archaeopteryx [an early toothed bird] was almost a perfectly juvenilised version of an ancestral dinosaur.
“Evolution can work by perhaps making one or two alterations to a hormonal growth pathway, and that causes countless anatomical transformations,” says Bhullar. “Evolution is simpler and more elegant than it might initially appear.” The process by which a descendant keeps the juvenile look of an ancestor in evolution has its own technical term: paedomorphism,
Where to draw the line between dinosaurs and birds is arbitrary. Indeed birds are today placed in a group called maniraptors, which includes Velociraptor. “I would be surprised if any of the dinosaurs lacked feathers,” Bhullar adds. He includes the gargantuan plant-eating sauropods. Even these giants possibly kept some feathers, though in reduced form, analogous to the way elephants evolved less fur for heat loss.