Consider the lowly roundworm, or nematode. A very primitive form of animal life, but a very successful one, ubiquitous around the world. They live everywhere, from the bottoms of the oceans to the tops of trees, and they can even be found in near-solid rock in deep mines. Many also live parasitically inside plants or animals. They account, by number, for around four fifths of all multicellular animals on Earth. There are tens of thousands of different species.
Unlike advanced worms, such as the earthworms in your garden, roundworms do not have a circulatory system — they have no heart and no blood. They absorb oxygen only through the outside surface, and it has to diffuse from cell to cell. This generally limits them to quite small sizes — larger than single-celled organisms, but not by a lot. Common sizes are somewhere around a millimeter long and a tenth of a millimeter thick, though in certain protected environments they can expand greatly in length. They can also be much smaller — as thin as five microns, which about the thickness of the individual strands making up a cobweb or a silk cocoon.
They lack many bodily features besides a circulatory system. They have no bones, of course, and no lungs or gills. They also have no eyes or ears, and almost no brain. They sense the world only through their permeable skins (or rather, cuticles). They do have a gut through which food passes from one end to the other, though the early larval stages may absorb food as well as oxygen directly through the cuticle. If you look closely enough, there’s also something in there which functions like a kidney. And for internal organs, that’s pretty much it — the only other thing they have which clearly resembles anything familiar from the viscera of higher animals is a gonad, which often fills about half of the body cavity. (They’re usually hermaphroditic.) In short, their bodies are so rudimentary and simplified that they make a dust mite look like a miracle of advanced complexity.
Let’s look at a specific example, namely the most thoroughly studied nematode, Caenorhabditis elegans. It lives in soil, and is a typical nematode size, one millimeter long. It has only about a thousand cells in its body. To be specific, it has exactly 959 cells in adult hermaphroditic form, not counting sperm and eggs. (Certain rare individuals grow up to be male only; these have exactly 1031 cells.) Each and every cell has an individually programmed shape, location, and role — a condition called eutely, which is common in similarly tiny organisms with small numbers of cells, such as rotifers and tardigrades.
It can be informative to look at how they are divided up. The main length of gut (not counting the specialized bits at each end), though a large part of the animal’s mass, uses only 48 cells — they’re very large and blocky compared to others in the body. The cuticle is around 200 cells, and the musculature which allows it to wriggle forward consists of 95 cells. The interesting bit is that of the 959 adult cells, 302 are neurons. That may not sound like much when even a lowly fruit fly has a quarter million, but a single neuron can do quite a lot of information processing. Despite its completely fixed neural wiring, this roundworm is capable of rudimentary learning and memory.
Those nerve cells are what make this otherwise very primitive type of organism significant. Because despite the lack of most everything we take for granted in an animal body, the fact that it has nerves and muscles means that evolution has already produced everything that’s really needed for the development of advanced creatures. The toolbox already has all the essential parts to create intelligent life, in a way that is not true of, for instance, a sponge. In terms of evolution, the progress from the origin of life to the achievement of intelligence is, in a roundworm, already like nine tenths complete. I’m almost tempted to say that you could practically coast the rest of the way — that once any animal has reached the point where it has muscles coordinated by nerves, the arrival of high intelligence at some later time is scarcely a surprise. Okay, that’s exaggerated, but really, it no longer requires any great leap — there’s a clear path forward that requires only slow and plodding improvement. If there’s any astonishing miracle in the process, it’s something that happened at a much earlier stage.
If you doubt this, I will mention that a lot of the useful bodily features that are absent in these worms are already part of the evolutionary toolbox at a wormlike level. Though a C. elegans has no hard body parts, some other roundworms do — they’ve got teeth. And rotifers can have rigid external skeletons, and they have chewing mouthparts. Tardigrades (water bears) have legs like a caterpillar, including claws at the tips. And they have eyes! Rotifers can have rudimentary eyes too, which might contain only a single retinal cell. It’s not at all surprising that a beginning like a rotifer or tardigrade could later produce an animal like, say, a tiny crustacean. Or that a worm could develop into a creature like a lamprey or hagfish — something which resembles the ancestors of vertebrates. Give something like that a hinged jaw, and it’s well on its way to producing something like a shark. Give that the ability to extrude bony protective plates like a sturgeon, and you’re well on your way to fishes with an articulated backbone. And so on until you get to apes and humans.
Looking the other way, a roundworm hardly seems very remarkable as an advancement relative to, say, a hydra. It is much better able to move purposefully through its environment, and that’s about it. And a hydra, in turn, is not really much more sophisticated in its behavior than many protozoans are — the only big advancement is that it is multicellular, with the muscular and nervous functions reasonably well separated into distinct cells. This allows them to become arbitrarily more complex, whereas a single cell probably can’t grow beyond a quite short list of adaptive behaviors.
Put this all together, and we see why most of the history of life on Earth was occupied with the slow process of perfecting single cells. Once the most basic multicellular animals developed, and became capable of movement… at that point the slow crawl forward became a race.
It’s not done yet.