This illustration from a 14th-century Dutch encyclopedia on animal life shows that scientists have been thinking for centuries about the transition from fin to limb.
Nature blooms / National Library
By Elizabeth Pennisi
Almost 700 years ago, Jacob van Maerlant, a Dutch poet, envisioned a fish for life on the land: it ran out of arms to hoist ashore. Three genetic studies make his fantasy seem strikingly tentative. Together, the studies indicate that the aquatic predecessors of four-limbed terrestrial animals, or tetrapods, were just as well prepared as the Dutch fantasy fish. They were pre-equipped with genes that could be used to make limbs, efficient air-breathing lungs, and nervous systems geared to the challenges of life on land.
“All these studies tell us that the origin of quadrupeds was something that was waiting to happen,” says Borja Esteve-Altava, an evolutionary biologist at Pompeu Fabra University in Barcelona, Spain. Genetic: “Everything needed was already there” before vertebrates came ashore almost 400 million years ago.
Fossils reveal the outlines of the story. Fish with fleshy fins supported at their base by a single bone, known as lobe fin fish, moved in shallow water about 375 million years ago. About 5 million years later, some of the lobes on terra firma crawled. The first fish to land fin must have had at least all the physical characteristics and genetic adaptations to do so, but researchers have not worked out how and when they are equipped for the change. “The big question of how such a big morphological shift actually took place remains very much in play,” says Peter Currie, a developmental biologist at Monash University.
In the trio of studies published last week in Cell, genes in living fish have taken the place of fossils as a way to look back in time. One set of clues comes from studies of mechanized zebrafish, a favorite model to study evolution. M. Brent Hawkins, then a student at Harvard University and now a postdoc, was shocked when he discovered zebrafish mutants with two legs resembling the forelegs of terrestrial animals in their fins, complete with muscles, joints and blood vessels. The finding is “quite spectacular,” said Marie-Andrée Akimenko, a developmental biologist at the University of Ottawa.
Two mutated genes, vav2 and waslb, was responsible for the transformation. Both genes encode proteins that are part of a pathway that controls the activity of Hox11 proteins, regulatory molecules that lead to the formation of the two forearms in mammals, among others. In fish, other proteins normally suppress Hox11 and prevent the bones from forming. But the mutations, which Hawkins recreated with the help of gene editor CRISPR, reactivate the way. The ‘landmark’ finding is ‘changing the paradigm of limb development and evolution’, says Renata Freitas, a developmental biologist at the University of Porto in Portugal.
Other genetic clues come from living representatives of ancient fishing lines. Only two groups of fishfish live today: lungfish and coelacanths. About 400 million years ago, they deviated from the line of fish that gave rise to tetrapods 30 million years later. Today’s oceans are mostly populated with species from another group that originated about 420 million years ago: the fish with a jet fin, so named because the fins are supported by slender spines.
Evolutionary geneticists Guojie Zhang at the University of Copenhagen and Wen Wang of the Northwestern Polytechnic University in Xi’an, China, and their colleagues sequenced the genomes of the African lungfish, which branched off early from other lobe finfish. The researchers also have the sequence of the bichir, a long, air-breathing, jet finfish that lives in the shallows of tropical African rivers, as well as the American paddlefish, the bow finch and the crocodile gar. All are fish with a jet fin that developed much earlier than teleosts, the group that dominates the world’s waters today (see diagram below). The researchers knew when each of these genera branched off from others, and were able to deduce when and where certain genes first appeared on the fish pedigree.
The long swim to land
The foundation for earthly qualities such as limbs and lungs is laid deep in the fish family tree. Genes for such traits found in fish fish as well as jet fins also had to occur in their common ancestor.
X. BI ET AL., CELL, 2021, DOI.ORG/10.1016/J.CELL.2021.01.046, Adapted by N. DESAI /SCIENCE
None of the successive fish are on the exact branch that led to tetrapods. Yet everyone has a lot of the genetic equipment needed for life on land, including most of the genes and regulatory DNA needed to build limbs. For example, all fish that are sequential have a regulatory element that helps form synovial joints, making fins and limbs flexible and essential for rural movement. The fish also has 11 genes that are needed to build lungs and that work in the bichir’s lungs in the same way as in humans. One is for a pulmonary wetting agent, a lubricating secretion that helps the lungs expand and contract. Both the fish with a ray fin and a lungfish with a lobe fin also have a regulatory element that helps shape the right ventricle of the heart to deliver oxygen more efficiently.
The findings show that ‘many things we think are only in terrestrial animals are fish,’ says Gage Crump, a developmental biologist at the University of Southern California. Finding all the genes in lobe fin or ray finfish means that the genetic pathways must have occurred in their common ancestor about 425 million years ago. “It’s amazing that some of these elements are so preserved for such a long evolutionary time,” Zhang says. (Teleosts, on the other hand, lost much of the DNA that prepared early fish for life on land, apart from the Hox11 orbit, the team reported.)
The genome of the lungfish offers a look at later adaptations on the path to earthly life. It contains additional pulmonary surfactant genes that the fish with ray fins do not have, as well as DNA to specify five toes, connect nerves to limbs and to make the brain sensitive to respond quickly. It was previously thought that all those genes are unique to squares.
To put it all together, Wang and Zhang think that the transition to land involves three important steps. The ability to breathe air appeared about 425 million years ago in the common ancestor of ray and lobster fish. Then surfactant genes, new nervous system genes and other innovations enabled fish fish to leave the water temporarily. Eventually, after the African lungfish tore off the lobes, the common ancestor of land vertebrates obtained other respiratory and locomotive refinements necessary to live out of the water.
Instead of building new structures and genetic pathways just as vertebrates moved to the land, evolution seemed to be sparing by using existing genes to adapt to the opportunities offered by rural habitats. ‘[The studies] shows the extent to which the transition between fish and tetrapod has been achieved by adapting existing molecular systems, rather than creating new ones, ”says Per Ahlberg, a paleontologist at Uppsala University.
There are still gaps in understanding how fish land, but the new studies “bring us closer to the living biology of the fish-tetrapod transition,” Ahlberg adds. Van Maerlant will be happy.
* Update, February 10, 12:30 p.m. This story, first published on February 4, has been updated to include information on two additional articles on genomics to understand how vertebrates adapt to life on land.