In 1859, Charles Darwin introduced the term “living fossils” to describe organisms exhibiting minimal species diversity or physical deviations from their ancestors as seen in the fossil record. A groundbreaking study from Yale researchers, published in the journal Evolution, has shed light on the biological mechanisms underlying the existence of living fossils in nature.
Focusing on gars, an ancient group of ray-finned fishes epitomizing the concept of living fossils, the study reveals that gars possess the slowest rate of molecular evolution among all jawed vertebrates. This sluggish genomic evolution implies that their genetic makeup changes at a significantly slower pace compared to other animals.
Linking this observation to the phenomenon of hybridization—where two distinct species produce viable offspring—the researchers propose that the slow evolution rate of gar genomes is a key factor contributing to their low species diversity.
Thomas J. Near, the senior author of the study and a professor of Ecology and Evolutionary Biology at Yale, highlights the groundbreaking nature of the findings: “We show that gars' slow rate of molecular evolution has stymied their rate of speciation. Fundamentally, this is the first instance where science is showing that a lineage, through an intrinsic aspect of its biology, fits the criteria of living fossils.”
The study speculates that gars possess an exceptionally robust DNA repair mechanism, enabling them to rectify somatic and germline mutations—DNA alterations occurring before and after conception—more efficiently than most other vertebrates.
If substantiated, these findings could hold significant implications for human health, according to Near. “Most cancers stem from somatic mutations that reflect deficiencies in an individual's DNA repair mechanisms,” he explains. “Should further research confirm the extraordinary efficiency of gar DNA repair mechanisms and uncover the underlying mechanisms, we may explore potential applications to human health.”
The structural resemblance of the seven extant gar species to their Jurassic ancestors from approximately 150 million years ago underscores their status as living fossils. One of the two major living gar lineages traces back to the middle Cretaceous period, dating as far back as 100 million years ago.
Analysis of a comprehensive dataset comprising 1,105 exons—the coding region of DNA—from 471 jawed vertebrate species revealed that gars' DNA evolves up to three orders of magnitude more slowly than any other major vertebrate group. Moreover, the study identified a correlation between the slow rate of molecular evolution and the slow rate of speciation in gars.
Chase D. Brownstein, the lead author of the study and a graduate student in Yale's Department of Ecology and Evolutionary Biology, elaborates: “The slower a species's genome is mutating, the more likely it is that it will be able to interbreed with a separate species that it's been genetically isolated from over a long stretch of time.”
By examining instances of hybridization between alligator gar and longnose gar—two distinct species with a common ancestor dating back at least 100 million years—the researchers discovered that these species continue to produce viable and fertile hybrids. This constitutes the oldest known parental split among all organisms capable of producing viable, fertile hybrids, surpassing the previous record by approximately 60 million years.
The study concludes that gars' slow rate of genetic mutation impedes both speciation and the evolution of new observable characteristics, contributing to their status as living fossils. The researchers emphasize that these findings not only enhance our understanding of biodiversity but also offer potential insights for medical research, potentially improving human health.
The study was co-authored by Daemin Kim and Oliver Orr from Yale's Department of Ecology and Evolutionary Biology, Daniel J. MacGuigan from the University of Buffalo, Liandong Yang from the Chinese Academy of Sciences in Beijing, Solomon R. David from the University of Minnesota, and Brian Kreiser from the University of Southern Mississippi.
Source: Yale University