Written by El Hebert '24
Edited by Alyssa Steinbaum '23
This insect neuron, lit up by chemical dyes, shows some of the complexity of these cells. NICHD / N. Gupta, hosted on Wikimedia Commons.
The seat of your conscious experience, the center of your senses and your willpower, looks something like a desert weed: a gnarled, blobby tuber of a brain, with deep roots branching below. But your nervous system may be the most complex object on Earth. How did such a structure arise from nothing? Scientists have begun to piece together the origin story by focusing on its basic unit, the neuron.
Anyone who observes a neuron, or nerve cell, can see that these cells are special. Where most animal cells organize themselves in simple shapes, a neuron tapers, winds, and forks, as befitting the body’s communication highway.
The core of the cell bears a tree-like crown of reception antennae to gather input from the neuron’s neighbors. On the other side, the long, thready axon acts as a cable, conducting messages in the form of electrical pulses. These tiny flashes of information zap to their targets: the synapse, or endpoint of the axon, which acts as a connection site with a muscle or another neuron. Here, chemical messengers relay the signal across the gap to be picked up and carried on.
At least a hundred billion of these beautiful cells tangle together to form your brain, with many more carrying information between that center and the rest of your body. Neurons make you human. But for all their importance, they’re still single cells - ghosts in the evolutionary record. New technology and fossil discoveries have started to clear the fog, though. First, they can answer a fundamental question: why grow a nervous system at all?
Despite the industrial warehouse’s worth of cellular machinery behind a neural impulse, these signals travel across the body in mere instants. If you’ve ever reflexively jerked your foot away from a thumbtack or a Lego lying on the ground, you’ve experienced the result of a nerve signal leaping from your foot to your spinal cord at up to 100 meters per second (it takes only one axon, as long as your leg, to carry that signal) [1]. And while engineers struggle to make computers recognize street signs, consider the speed with which you process the moving road as you drive.
It was this effortless speed, researchers suspect, that first drove the system’s evolution. Today, it makes neurons a must-have for all familiar animals - creatures who scuttle and strike and flinch, like humans, roaches, and anemones. So it went for our ancestors, too.
Around 540 million years ago, animal life exploded with diversity and invented a bizarre new concept: eating each other to survive. Fossils reveal how their bodies rose to the challenge, growing eyes, teeth, and paddles, with neurons to orchestrate the whole operation. In China, shrimplike fossils around 500 million years old already have central nerve cords similar to their descendants today [3]. But because soft tissue doesn’t preserve well, these traces are exceedingly rare, and only show the broadest neural thoroughfares. Nothing remains to lay out the murky origins of the cells themselves.
For that, scientists must turn away from ancient creatures and examine the modern. By mapping traits across the tree of life, researchers hope to determine when and how they first branched off. This is the field of comparative phylogenetics. For example, while birds, mammals, and fish all have bony skeletons, their older relatives, such as sharks, use cartilage, and the majority of other animals are squishy. So we can guess that the internal skeleton first evolved among the ancestors of sharks and hardened into bone sometime after the sharks split from the classic fish. Phylogeneticists can treat an evolutionary story like a puzzle, fitting together clues from the unlikeliest creatures.
Of course, to use phylogenetics, we need to know who is related to whom in the first place. The family tree of fish and sharks is well-mapped, making the skeleton’s story relatively clear. Neurons, on the other hand, likely arose at the earliest divergence of animal families, the splits that separated you from a jellyfish. However, among the biology community, those branches are in turmoil.
It was once assumed that the first animal looked something like a sea sponge - a placid filter feeder with no need for fancy neurons - and that the swimmers and burrowers split from this ancestor, forging ahead to greater complexity with every new branch. Thus today’s sponges lack nerve tissue, while jellies, the second most ancient lineage, have it.
But now, with genetic evidence, the waters are muddied. The comb jelly, a sophisticated predator with neurons aplenty, may better resemble the ancestor of all animals, meaning that the sponge family settled to the bottom and lost its complicated parts! Alternatively, the position of the sponges and comb jellies may not matter so much. Perhaps neurons arose sometime after the first few major splits, evolving independently in each animal family [2].
Whatever the answer, sponges are a favorite target for evolutionary biologists concerned with these questions. Whether they lost their neurons or never had them in the first place, they still hold important clues to how animals got by without. In fact, they maintain the same chemical messaging machinery we know from our own nervous system.
These molecules were recently found to whir with activity in certain digestive cells. In a study published in Science this November, highly detailed images show how the cells reach out to touch the water-pumping apparatus of their neighbors. There, the messenger chemicals may help nudge the pumps into coordinated action, the same way a neuron communicates with its target through the synapse. It seems that such early chemical synapses were already in use in primitive animals, just waiting to be electrified [4].
Riotously busy chemical synapses, like the one in this illustration, likely evolved from simple beginnings as cell-to-cell coordinators. The tiny, unclustered dots here represent messenger molecules crossing the gap between cells. Maria Voigt and PDB-101, hosted on Wikimedia Commons.
Electric communication may sound like a sophisticated leap, but in fact, it’s probably many times older than the synaptic side of things. The “channel” proteins that let electric charge into and out of the cell are shared by bacteria, the most distant of our relatives. These molecules are up to four billion years old.
What use could an early bacterium have for an electrifying protein? Quite a bit, it turns out. Sodium, potassium and calcium, the chemicals that power nerve signals today, give the sea its saltiness. Life was born swimming in the stuff and had to keep a careful hold on the ions flowing into and out of their single-celled bodies. But an innovative cell can find many more uses for an ion channel than that, especially in communication. Today, when huddled together in a collective of scum, bacteria use their potassium channels to send electric waves across the colony and synchronize their feeding [5].
Eventually, these two functions - chemical and electrical signaling - must have combined in one single, specialized cell. Researchers generally agree that an epithelial cell could do the trick. This tissue forms your skin and the lining of all your internal organs, and, early in development, pinches off to grow into your first nerves. (In jellyfish, epithelial cells can still produce electrical impulses [2].) We don’t know when this change happened, or even how many times, but it stuck around. Life was speeding up. As animals began to make split-second decisions, neurons were just what they needed.
These are the broad strokes of the neuron’s origin story, pieced together from clues left in unlikely creatures. Whole chapters remain out of reach - how, for example, did those first cells get so long and wirelike? - but what we do know still offers a glimpse into evolution’s workings. It’s a resourceful process, combining old tricks in new ways. It’s a convoluted process, capable of unraveling what it makes. And it’s a beautiful process, generating complexity from simple beginnings. As you interpret these words with your human brain, remember the sponges and jellies and bacteria of the deep past, the humble innovators who made it all possible.
References
[1] Bullock TH, Horridge GA. Structure and Function in the Nervous Systems of Invertebrates. San Francisco: W. H. Freeman; 1965, vol I pp.150-151 table 3.4.
[2] Kristan Jr WB. Early evolution of neurons. Current Biology. 2016 Oct 24 [cited 2021 Dec 12];26(20):R949-54.
[3] Yang J, Ortega-Hernández J, Butterfield NJ, Liu Y, Boyan GS, Hou JB, Lan T, Zhang XG. Fuxianhuiid ventral nerve cord and early nervous system evolution in Panarthropoda. Proceedings of the National Academy of Sciences. 2016 Mar 15 [cited 2021 Dec 12];113(11):2988-93.
[4] Musser JM, Schippers KJ, Nickel M, Mizzon G, Kohn AB, Pape C, Ronchi P, Papadopoulos N, Tarashansky AJ, Hammel JU, Wolf F. Profiling cellular diversity in sponges informs animal cell type and nervous system evolution. Science. 2021 Nov 5 [cited 2021 Dec 12];374(6568):717-23.
[5] Prindle A, Liu J, Asally M, Garcia-Ojalvo J, Süel GM. Ion channels enable electrical communication in bacterial communities. Nature. 2015 Nov [cited 2021 Dec 12];527(7576):59-63.