Replanting the Tree of Life
By Karen Zusi
BU News Service
Charles Darwin first represented his theory of evolution with an image of the tree of life in 1859: “Of the many twigs which flourished when the tree was a great bush,” he wrote in The Origin of Species, “only two or three, now grown into great branches, yet survive and bear the other branches.” From a single root at the base of the animal kingdom, species have split and changed—some with more success than others—in new patterns represented by these branches. Closely-related species such as humans and chimpanzees share a branch until it splits into twigs at the top of the tree. Distant relatives separate much earlier; the branch for insects divides lower on the trunk than the branches joining primates and canines.
The study of these evolutionary relationships is called phylogeny, and the field involves more than answering fun questions about our relatives in the animal kingdom. By understanding evolutionary relationships, scientists can test drugs on mice, use worms to study theories about nervous system development, and breed fruit flies to investigate changes in certain genes. These models only work if scientists know how closely related these organisms are.
The placement of different branches on the tree used to be based on what animals looked and acted like, with simple animals like sponges and jellyfish rooted at the base of the tree and complex ones emerging as new growth near the crown. Humans have always occupied the branches at the top. However, the technology that sequences and compares genes has sprouted a new tree of life that, rather than being firmly rooted, tends to shift and sway—and gives rise to the idea that our ancestors may not have been so simple after all.
One of the first major shifts to the animal tree came in 1997, when James Lake at the University of California, Los Angeles, and his colleagues published an evolutionary tree that contradicted a fundamental organizing principle of evolution. Scientists had previously classified species based in part on the body cavity between the digestive organs and the outer body wall. They divided animals into one of three groups depending on whether they had a body cavity, a partial cavity, or lacked one altogether. Animals like jellyfish, sea anemones, and tapeworms don’t have one; internally, they are organized by layers of compressed tissue. Nematodes, the roundworms like hookworm and pinworm, have a partial body cavity, and complex animals like humans have a well-organized space for their organs.
Lake showed that this organization scheme using internal structures didn’t hold up when examined with DNA. The nematode branch in particular had been difficult to place on the tree because these creatures can change rapidly over generations, faster than other animals. Accumulating many genetic differences over a short period of time makes them look unrelated to groups that they’re actually close to. Lake specifically used a slowly-evolving nematode species to show that they should be grouped with arthropods—animals including fruit flies, scorpions, and lobsters. Arthropods have a fully-developed body cavity, and nematodes don’t, which tossed that organizing principle out the window. Instead, the data suggested that nematodes came from an ancestor with a body cavity and then lost it through evolution, a “regression” from complex to simple.
Lake still had to answer the question of what other features could connect nematodes and arthropods. Arthropods were previously grouped with the annelids—worms like leeches and earthworms—because both types of animals are segmented in addition to having developed body cavities. According to Lake, it was an office-chair epiphany that cemented the change for him: “I still remember sitting here at the desk going, ‘Why could they be related? What do they have?’” he says of the 1997 study. “And then I said, ‘Oh my god, they molt.’ It was like a magic moment. I sat back in my chair, and I laughed, maybe like a child, and I said ‘That’s it!’” Both arthropods and nematodes molt an outer layer of hard skin while growing. Lake’s change rearranged the tree of life’s branches, creating a new one based on this shared trait.
Gonzalo Giribet at Harvard University headed a research team at the same time that Lake published his work, and reached the same conclusion independently. Instead of using one slowly-evolving nematode species, Giribet increased the overall number of nematode species in his analysis to six. With the larger dataset, his data still showed the same results. Over the next twelve years, as genetic technology improved, other research groups across the country were able to replicate Lake’s results and create a growing body of evidence for the rearrangement. After a slow start, by 2009 the scientific community had overall accepted the change. During the 150th anniversary year of Darwin’s Origin of Species, the Royal Society of London, the world’s oldest scientific academy, held an international biology conference where every speaker endorsed Lake’s results. “By that time I knew everything was done,” he says. “It was really gratifying—it had been a big fight.” Lake was awarded the Darwin-Wallace Medal by the Linnean Society of London in 2011 for the new animal grouping.
Just as Lake’s results were gaining widespread acceptance, another study was rocking the base of the evolutionary tree. Down at the roots, sponges have traditionally been considered the most primitive animals, lacking organs, nerves, and muscles. They live underwater, stuck to rocks, and feed by drawing in water to catch food particles. Sponges are so different from any other type of animal, and so simple, that until recently it seemed almost impossible that anything could have come before them. Surely, these creatures were basic relics of a distant past.
In 2008, Casey Dunn at Brown University led a research group upending that perspective. Dunn’s team provided the first genetic information from a group of animals called ctenophores. These are the “comb jellies”—marine predators that look like blobs of jelly and swim using hair-like projections on their bodies. Until 2008, ctenophores were the only major animal group that scientists had never genetically examined. “We needed a sequence to address this void in such a critical part of the tree,” comments Andy Baxevanis, head of the Computational Genomics Unit at the National Institute of Health who was involved in later research. When Dunn’s team finally sequenced the ctenophore genes and compared them to the rest of the animal kingdom, they found that the ctenophores were so different from everything else that they had to put them at the base of the tree—in place of the sponges.
Ctenophores look more complicated than sponges: they have a nervous system and muscles, and they can move. You’d think they would occupy a higher branch than the sponges, but the story is more complicated. Scientist who have done the genetic analyses think the sponges may have had the precursors to those structures but lost them in an evolutionary “regression”—or never used the building blocks at all. “We find that somewhere way back in their distant evolutionary path, the sponges lost their ability to develop a nervous system,” says Baxevanis. Perhaps sponges simply evolved on their own to be the best filter-feeders in the world, discarding physiology they didn’t need.
Since 2008, more data has been published—including whole genomes—that continue to support the ctenophores’ early split from the rest of the animal kingdom. Researchers have discovered, among other things, that ctenophore muscles are genetically different from every other member of the animal kingdom. However, these conclusions are still hotly debated among the scientific community, especially by sponge researchers. “As soon as ctenophores became the sister group to everything else, everyone went crazy,” says James Ryan, first author of a 2013 paper expanding on Dunn’s research. The only resolution will lie in analyzing more information from different species.
Regardless of where the ctenophores end up on the tree, genetic sequencing has shown that the philosophy of evolution as a ladder of complexity is anthrocentric and outdated. Species gain and lose traits based on what helps them survive and reproduce in their environments, and sometimes that includes losing features that their ancestors had. It’s not really fair to call these changes de-evolutions—animals simply adapt as-needed so they can devote their energy to activities that matter. The shape of the animal tree of life just may be more complex than scientists ever imagined.