Evo-devo is shedding serious light on our biological origins
Science | by Gregory Beatty
Charles Darwin is credited with discovering the theory of evolution which upended centuries of Christian belief that life on Earth was divinely created. But decades before he wrote On the Origin of Species in 1859, scientists such as Étienne Serres and Johann Friedrich Meckel were uncovering evidence that suggested humans and animals had common ancestral roots.
They were spurred on in their research by the remarkable similarity found in embryos of many different species such as chickens, lizards, non-human primates and humans, says University of Saskatchewan professor Julia Boughner, who teaches anatomy and cell biology. “There’s a classic anecdote about someone who pickled embryos from different species and didn’t label them properly, and when he went to look at them he couldn’t tell which was which.”
As later stage embryos and newborns, the animals were radically different. Yet early in their development, they were eerily alike. To Serres and Meckel, that suggested a common origin which was outlined in a Recapitulation Theory that held that “higher” animals went through stages where they resembled animals lower down on Christianity’s “great chain of being”.
That turned out to be wrong. But their core observation was still valid, says Boughner. “There’s something called the phylotypic stage where embryos all seem to go through a bottleneck where everything is similar, then they begin to diversify. The early stage of pre-natal development reflects deeply conserved evolutionary ancestry. The later stages reflect evolutionary novelties that distinguish different species.”
DARWIN & MENDEL
While that was the start of the discipline that today is known as evolutionary developmental biology (or evo-devo for short), how the process worked was still a mystery.
Darwin, via his idea of natural selection, which described how life was continually adapting in response to changing environmental conditions, made the first breakthrough by identifying the mechanism that drove evolution.
Gregor Mendel was conducting research into genetics at the same time, but it took several decades for his work to be accepted. Once it was, it explained how animals that were successful (in the sense of surviving long enough to reproduce) were able to pass on traits to their offspring.
Animals, including humans, have many thousands of genes encoded in their DNA (around 23,000, in our case). Only a few guide embryonic development, though. Scientists call them “tool-kit” genes, and when they kick in they trigger a cascade effect in other genes.
“Say you look at a tetrapod, which is an animal with two arms and two legs,” says Boughner. “You can look at tons of animals, and they all have arms or forelimbs and legs or hindlimbs. But they look really different. That’s an example of where, after the initial body plan was laid out to have four limbs, you get genetic instructions that say how many digits you have, or whether you’re going to have hooves or webbed feet.”
It might seem impossibly complex — and to hardcore Christians, it is, as they attribute it to God or Intelligent Design — but remember, life’s existed on Earth for over 3.5 billion years, so nature’s had plenty of practice at reproduction. It isn’t infallible, though, as mutations do occur. Most times, they render an embryo unviable, and it’s either aborted in the womb or the animal dies shortly after birth. Occasionally, though, a mutation proves advantageous, and it gets passed on.
It’s important to remember, too, that life doesn’t exist in a vacuum. Over its 4.5 billion year history, Earth has gone through many wrenching changes in geographic makeup, climate and habitat conditions. And those changes have exacted a heavy toll.
“If you look at the fossil record, I’m sure there are many more species that have gone extinct than are alive now,” says Boughner. “Some, like dinosaurs, were tremendously successful in their day. But eventually the environment changed and only a few were able to adapt, and now we call them birds.”
HOMINIDS & HUMANS
Boughner’s speciality at the University of Saskatchewan is evo-devo anthropology. Her entry into the field, she says, came when she became interested in the interplay between human biology and culture.
“The catalyst was looking at a bunch of cast fossil skulls in an undergraduate lab from about four million years of hominid evolution. There was a very noticeable trend from big jaws and teeth, especially really big molars, in australopithecines and other early ancestors. Then when you get closer to the start of our species, you see this decrease in size of the jaws and teeth.”
The decrease coincided with a period when we were starting to make tools and use fire, says Boughner. “This led me to think ‘Were teeth and jaws getting smaller because we’d invented stone tools to process food and fire to cook it and didn’t need them anymore?’
“Or was there some other selection to get smaller teeth and jaws because it takes energy and resources to grow them? So if we were short on calories, maybe those systems were being selected to become smaller, and that was a catalyst for cultural evolution where we needed to make tools and cook food because we no longer had massive jaws and teeth.”
Studying different skulls, Boughner determined that the downsizing had occurred in tandem. “It wasn’t like the jaws we’re getting smaller and the teeth were staying big, or vice versa. They were doing it together. I was fascinated with how that was happening.”
Genes drive the development process, so that led her to study molecular biology. Then at the tail end of her PhD research, she stumbled on evo-devo. “It had been around for 200 years, but it seemed to be really hitting its stride in the early 2000s,” she recalls.
Technology has played a vital role in advancing evo-devo. Gene sequencing, for instance, is much faster and more precise then it used to be, which allows researchers to hone in on genes and identify the role they play in development.
“Imaging is huge too, including looking at live embryos,” says Boughner. “One of the biggest problems in studying development is seeing it happen. Before, people had to rely on embryos that were no longer alive.
“That takes the embryo out of the context in which it grows, and you can’t puzzle together a series of developmental stages to figure out what’s going on. Now, there’s imaging techniques that help you track development in a single individual rather than having to hodge-podge a bunch of embryos together.”
While fascinating from a purely scientific perspective, evo-devo is yielding real-world benefits, says Boughner. “The information being generated is incredibly valuable in understanding the origin of congenital diseases. If we can understand how that works, we can maybe use that information to prevent it from happening, or treat it through corrective surgery after birth.
“There are all kinds of tissue engineering implications as well with transplants,” she says. “In the future, we might be able to print out a body part. Or maybe instead of using surgery to hook-up a body part we’ll be able to tap into our knowledge of gene expression and growth factors and grow a part inside the body.”
Climate change is another example of real-world relevance, says Boughner. “By studying the evolution and biology of animals we might understand how adaptable species are to change. If some embryos develop in a colder temperature versus a warmer temperature, it can change the animal’s physical characteristics, behaviour, even sex. This has immediate implications for what we’re seeing in natural populations around the world as the climate continues to warm.”
The bottom-line for Boughner is that evo-devo is just cool science. “I think it has huge potential in education. There are so many fascinating stories about animals. You can go into a grade one class and talk about how a chicken develops and even ‘window’ an egg where you cut a hole and watch the embryo under a microscope.
“Eventually, it hatches into a chick. Then you can tell them that birds came from dinosaurs, and that they developed very similarly to this chick. So it’s a way to link the past to the present, and enable children to grasp our interrelatedness to other animals that both live in the world today, and those who used to live in the world but went extinct.” ❧