How did brains evolve? Sponges have cells that may be precursors of neurons

Sponges are arguably the only animals and so they lack a nervous system, however peculiar cells of their digestive chambers could also be evolutionary precursors of neurons

By Michael Marshall

Sponges lack something resembling brains, however they nonetheless might have performed a key function within the early evolution of the nervous system. A brand new examine finds that sponges include cells which have a few of the capabilities of neurons – and these could be the evolutionary precursors of true mind cells.

“The nervous system took place very early in animals and this transition is totally enigmatic to this point,” says Detlev Arendt on the European Molecular Biology Laboratory in Heidelberg, Germany.

Most animals have brains, or at the least neurons, the cells which are their constructing blocks. Neurons carry electrical alerts alongside their size and might talk with one another by releasing chemical compounds known as neurotransmitters, typically at specialised junctions often called synapses.

Nevertheless, sponges are the exception. They’re one of many oldest animal teams nonetheless extant – presumably the very oldest. And so they don’t have a nervous system.

“Sponges don’t have something that appears like neurons, synapses or brains,” says co-author Jacob Musser, additionally on the European Molecular Biology Laboratory. However his crew has discovered they could have precursors of this stuff.

Musser, Arendt and their colleagues studied a freshwater sponge known as Spongilla lacustris. They broke aside sponges and tracked particular person cells to see which genes have been lively.

This revealed that the sponges have been made up of 18 distinct cell varieties, every with a distinct sample of gene exercise. The crew then stained the completely different cells to determine the place they have been throughout the physique.

One cell kind stood out. The crew calls them “neuroid” as a result of that they had lengthy tendrils, resembling these of neurons. They have been discovered within the sponge’s digestive chamber and made contact with most of the different cells inside. Their gene exercise sample advised they have been secreting signalling chemical compounds, comparable to people who neurons launch at synapses to speak with their neighbours.

Arendt emphasises that the sponges’ neuroid cells aren’t neurons. “We nonetheless assume they don’t have a nervous system,” he says. However these cells could also be coordinating the actions of the digestive cells. “We see plenty of vesicles in these neuroid cells that will point out that they secrete one thing, which is a really sturdy indication for communication,” he says. “And we additionally know the type of molecules they could produce.”

Sponge biologist Sally Leys on the College of Alberta in Canada describes the single-cell information as “excellent”. However she is unconvinced by the crew’s interpretation. “I believe there’s no proof proven on this paper that these are something to do with a neuroid precursor in any respect.”

Leys argues that most of the genes the crew centered on are extensively utilized in advanced organisms and aren’t particular to neurons. The sponges could also be utilizing them for various functions. She needs to see useful research demonstrating that the neuroid cells truly behave the way in which the genetics suggests they do.

Arendt factors out that most of the genes and chemical compounds utilized by neurons are literally historical: they predate the evolution of multicellular animals and will be present in our single-celled kin. Later, the genes have been duplicated and a few variations turned altered, finally resulting in neurons which are extremely specialised for quick communication.

That step occurred early in animal evolution. The opposite candidate for the oldest animal group, the comb jellies, do have neurons – and they’re organised right into a web. “I’d be nice with calling {that a} mind,” says Arendt. Arguments have raged over the previous decade over whether or not sponges or comb jellies are the older group. Arendt is inclined to consider that sponges, being easier, are older.

What ought to we make of the truth that the neuroid cells have been within the sponges’ digestive system? Arendt and Musser say there’s rising proof that some neurons developed from digestive cells. “There are neurons in a wide range of completely different animals that share plenty of options with digestive cells and even come from a few of the identical embryonic precursors,” says Arendt. Nevertheless, different neurons have extra in frequent with cells that contract, akin to muscle cells, and should signify a separate origin of neurons.

“A variety of researchers would have stated a number of years in the past neurons in all probability had a single origin,” says Musser. “That story appears to be altering.”

Journal reference: Science, DOI: 10.1126/science.abj2949