On Aug. 14, 2024, scientists from Uruguay University announced that a species of lungfish found in South America has claimed the title of the animal with the biggest genome sequenced to-date. The DNA of Lepidosiren paradoxa comprises a staggering 91 billion chemical letters or “bases,” 30 times as many as the human genome, researchers reported in Nature. Those 91 billion bases of DNA only contain about the same number of genes that humans have—roughly 20,000—with the rest consisting of noncoding, perhaps even “junk” DNA. By comparing this genome with those of other lungfishes, the researchers determined that L. paradoxa added the equivalent of a human genome to its DNA every 10 million years.

Sometimes called “living fossils,” lungfish are air-breathing lobe-finned fish believed to most closely resemble the most ancient common ancestors of all vertebrates. So, since 2018, Axel Meyer and Manfred Schartl, evolutionary biologists at the Julius Maximilians University of Würzburg, and their colleagues have worked to decipher the genomes of lungfish from three continents to better understand vertebrate evolution.

They first sequenced that of the Australian lungfish, whose fin bones resemble those in our limbs and those of other land animals. They then moved on to African lungfish, whose limb bones are much reduced. Now, they’ve tackled the South American lungfish, which also has reduced fins—and a genome twice the size of its Australian and African cousins.

Included in that massive genome are the organism’s genes, the instructions for its proteins written in four different chemical “letters” called nucleotides or bases, as well as codes for helpful bits of RNA. But the vast majority of L. paradoxa’s genome is composed of something else: repetitive stretches called transposable elements that can make copies of themselves and insert those copies back into DNA. About 90% of L. paradoxa’s genome consists of these “selfish” sequences, the researchers find, compared with about 40% of the human genome.

How did the lungfish’s genome become so cluttered with repetitive DNA? Comparing it with other vertebrate genomes revealed its growth goes unchecked because the South America and African lungfishes have lost key genes whose RNA and protein products put the brakes on transposable elements. “This must be a huge cost to the animal,” Meyer says. All but one of this lungfish’s 19 chromosomes is the size of the entire human genome, so it takes a lot of energy to copy that DNA. And the nucleus and cell that encase it must be bigger.

A larger genome can sometimes help an organism adapt to changing conditions, Garcia notes. When transposable elements land in or near a gene, they can alter the activity of that gene, which may be beneficial under the right conditions. These shifty bits of DNA genes can even be fodder for new genes entirely. However, “It can [also] become a sword of Damocles if it is not possible to regulate genes that ensure the viability of the organism,” she says. So, it’s unclear why the fish didn’t evolve new ways to control the spread of these so-called jumping genes. Meyer goes to far as to say, “These are fish that evolution forgot.”

L. paradoxa’s genome is not the biggest of all living things, however. That title was recently bestowed on the New Caledonian fork fern (Tmesipteris oblanceolate), with its 160 billion bases. And other plants, such as a rare Japanese shrub named Paris japonica, also have enormous ones.