Darwin viewed evolution as a slow, gradual process in which species accumulated small changes over time. New research in some marine worm species is challenging this view. According to this school of thought, called punctuated equilibrium, organisms remain stable for ages before undergoing a massive change, taking a large leap forward in evolutionary history. In this research, scientists have found that certain marine worms underwent a complete reshaping of genetic makeover 200 million years ago to suit life on land. Their entire genome was broken into fragments and randomly reassembled almost overnight, defying the usual view that evolution occurs slowly.
The theory of punctuated equilibrium arises from the lack of fossil records showing intermediate forms of certain animals and the absence of missing links. Fossil records are basically like an archive from which many pages have been torn off. The idea that sudden, rapid, and radical changes lead to evolution explains the lack of fossils: large changes occur in a small number of animals in a population, keeping them away from paleontological radar.
Now, a research team led by the Institute of Evolutionary Biology (IBE), a mixed research centre belonging to the Spanish National Research Council (CSIC) and Pompeu Fabra University (UPF), points to rapid and massive genomic changes in the transition of marine to land animals 200 million years ago. The genetic reorganization in marine annelids (worms) left the genome unrecognizable when they transitioned to life on land. This genetic mechanism can transform and revolutionize previous thoughts regarding genetic evolution.
The team sequenced a high-quality genome of various earthworms for the first time and compared them to other closely related annelids like leeches and bristleworms. After the team put together the genomic jigsaw puzzle, they could travel back 200 million years, to a period when the ancestors of the sequenced species were alive. According to Rosa Fernández, lead researcher of the IBE’s Metazoa Phylogenomics and Genome Evolution Lab, this period marks the transition of worms and vertebrates living in the ocean to migrating to land.
The team discovered that the marine worms broke their genome into hundreds of smaller fragments and reorganized them to suit life on land. The entire genome was broken down and reorganized randomly in a short span on the evolutionary timescale.
A drastic reorganization as this could have led to extinction; however, the flexibility of the chromosomes of these modern worms allowed the genomic changes to take place without any harm. Major changes that took place helped the worms adapt to life on land, like breathing air on land and tolerating sunlight. The genetic adjustments didn’t end with positional switches; they also involved joining separated fragments, creating new “genetic chimeras”.
This phenomenon of extreme genetic reorganization has been noted in cancer in humans. In human cancerous cells, the chromosomes break down and reorganize in a process called chromoanagenesis. Similar changes have been observed in earthworms. However, unlike humans, these changes don’t cause any disease in earthworms. This study on radical genomic mechanisms can have implications for human health.
The study has also reawakened one of our most vital scientific debates. “Both visions, Darwin’s and Gould’s, are compatible and complementary. While Neo-Darwinism can explain the evolution of populations perfectly, it has not yet been able to explain some exceptional and crucial episodes in the history of life on Earth, such as the initial explosion of animal life in the oceans over 500 million years ago, or the transition from the sea to land 200 million years ago in the case of earthworms,” Fernández notes. “This is where the punctuated equilibrium theory could offer some answers.”
The next step in this study is to conduct a larger investigation of the genomic architecture in lesser-studied invertebrates. This can shed light on genomic mechanisms that shape the evolutionary pathway in these animals.
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