Octopuses may have acquired some of their exceptional intelligence from the same evolutionary process humans went through, a new study suggests.
The process involved a sudden explosion of microRNAs (miRNAs) – small, non-coding molecules that control how genes are expressed. This increase may have helped octopus and human brains develop new types of nerve cells, or neurons, that were sewn together into more complex neural networks.
Octopuses and their close cephalopods relatives, such as squid and cuttlefish, have been a subject of fascination among biologists since the third century AD, when the Roman author and naturalist Claidius Aelianus noted their “clearly visible” characteristics of “mischief and cunning”. Octopuses have remarkable memories; excel in camouflage; are curious about their surroundings; have been observed using problem solving tools; and, the ripples of color that flash across their skin as they sleep are even thought to be dreaming.
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But the exact basis for how their minds developed such complexity independently of ours remains a fascinating puzzle. For example, the last common ancestor of humans with octopuses was a seafloor-draging flatworm that lived about 750 million years ago and possessed nothing but a rudimentary brain. A recent study has shown that jumping genes, known as transposons, could explain some of Octopuses’ cleverness. Now, a new study published Nov. 25 in the journal Scientific progress perhaps found another important piece of the puzzle.
“If you want to know more about the intelligence, or brain, of an alien, a good model for that is to study the Octopusstudy senior author Nikolaus Rajevsky, a systems biologist at the Max Delbrück Center for Molecular Medicine in Berlin, Germany, told Live Science. “The evolution of its complex brain, and the cognitive features that go with it, happened completely independently of us. So by comparing it with us, you can learn more about general characteristics that we share, but maybe you can also find things that the octopus has that we don’t.”
The researchers studied 18 different tissue types from dead common octopuses (Octopus vulgaris), analyze their RNA and compare it with the RNA of other cephalopods such as the California two-spotted octopus (Octopus bimaculoides) and short-tailed squid (Euprymna scolopes)as well as more distant relatives such as the nautilus and cnidarians.
RNA is a single-stranded length of genetic code that is transcribed from DNA to make proteins in cells, and is sometimes involved in regulating gene expression. Initially, the scientists believed that octopuses used specially engineered enzymes to edit their DNA for greater neuronal complexity, but what the tissue analysis revealed instead was a historic explosion in the number of different miRNAs preserved across multiple species of cephalopods; a number comparable to that of some vertebrates, such as humans.
MiRNAs are small pieces of RNA that bind to protein-coding RNA strands, regulating their activity and silencing the expression of certain genes. This allows the genome to be more precisely tailored to specific purposes, creating new types of brain cells that can be chained into more elaborate neural networks. The researchers found a whopping 51 new families of miRNAs that have survived in octopuses and cephalopods since their ancestral lineage split more than 300 million years ago, and octopuses alone gained 90 since their last common ancestor with other molluscs such as oysters – which there are had acquired only five.
“This is just spectacular,” Rajewsky said. “Octopus microRNA numbers skyrocket to reach levels comparable to complex vertebrate brains.”
The researchers also found that the octopus miRNAs are most abundant in nerve tissues in the developing brains of octopus pups — strongly suggesting that the RNA regulators drive the development of more complex cognitive skills.
The researchers emphasize that a direct link between miRNA numbers and advanced intelligence has not yet been directly proven, and that to establish this link scientists will need to complete follow-up studies on the cell types to which the new miRNAs map. By doing so, the scientists hope to not only find the things we share with the alien brains of octopuses, but also unearth parts of the octopus genome that can be used to develop better tools for editing our own.
“This is not, I think, completely crazy, because a lot of things have been discovered this way,” Rajewsky said. “For example, CRISPR-Cas9 doesn’t exist in our genome, but bacteria have it, so now you can use it to edit ours.”