“There is nothing in natural selection that allows you to predict any particular pattern that it might generate.” —Henry Gee
“Life is not oriented towards increasing complexity, nor is it fated to become ever more complex.” —Alexandre Meinesz
When most people hear the word aliens, as in life from other worlds, they immediately picture something that is strongly humanoid—upright, two legs, two arms, a trunk, one head, two eyes, etc. This limited line of imagination is almost totally due to science fiction writers and filmmakers. There is absolutely no scientific reason to assume that the humanoid form is a probable outcome for evolution on another planet, and highly intelligent humanoids are even less probable. Though it has obviously come about here on Earth, it certainly didn’t have to happen—it was just one of countless millions of possibilities that chanced to materialize from Earth’s evolutionary process.
In another article (Zeigler 2008), I argued from a biological perspective that intelligent humanoids were but one of many unlikely possibilities. In that article I also ventured some predictions of what we might expect to find in terms of life on another planet. These were mostly very general predictions such as photosynthetic organisms, predators, parasites, and so on. I did not then attempt any prediction of what those forms might have looked like, though my major argument was that to expect anything like a humanoid form would be a long shot at best.
Having thought more about that point (perhaps prodded by the inane History Channel series Ancient Aliens), I decided that if I were to venture any prediction of form for any supposed aliens, it might well be that of worms. Of course, the safer bet would be microbes, especially photosynthetic or chemosynthetic microbes. The first lifeforms in any evolutionary process would most likely be simple microbes of some type, and after any abiotically formed “soup” was exhausted as a fuel source, some of those microbes would have to evolve some way of producing energy-containing compounds (fuel) using either sunlight (photosynthesis) or chemical processes (chemosynthesis). Microbes, then, are the surest bet.
Beyond microbes, if any multicellular forms with three or more cell types evolved, and if those forms did not synthesize their own fuel (were heterotrophic, in scientific terms), we might roughly refer to those forms as animals, especially if some of them had powers of locomotion. So what might alien “animals” look like? The real answer is that we have no idea and will likely never know since this would most likely require the exploration of far distant solar systems—something extremely unlikely to occur, no matter how much some physicists talk about wormholes in space (pun not intended).
A far more likely (than humanoid) possibility for alien life would be something we would recognize as a worm. I will have to be a bit technical and use some names and terms most readers will be unfamiliar with but can easily look up if clarification is needed. Let me start by pointing out that out of an approximately 10,000,000 living species (and this is a conservative number) and close to a billion species to have ever lived on this planet, only one has achieved humanoid form and advanced intelligence. Throw in chimps, gorillas, and the few other apes if you feel so inclined, but that still only comes to half a dozen or so such species, and of course these are all closely related recent forms that descended from one common ancestral species, which itself might easily not have evolved at all.
Worms would be a far more likely bet based on what we know of earthly life. And what other source can we draw from? Why worms? A general rule of thumb in evolution (not without exceptions) is that simple forms precede complex forms, at least in the initial stages of evolution. Worms are the simplest animals with an anterior end and a right and left side, something the simpler sponges and jellyfish lack. The Animal Kingdom is now divided into some thirty-plus major divisions called phyla. Humans are in the Phylum Chordata along with all other vertebrates, most of which are fish, as well as some invertebrates such as sea squirts and lancelets—technically no worms here, though later I will mention some Chordates that did evolve elongated limbless bodies. But of the other phyla, a surprising number contain either all worm forms or at least some worm forms.
An animal phylum is a grouping that includes all the animal species in one very old line of descent, going back in time to the very beginnings of the animal kingdom in the Precambrian era (more than 540 million years ago). In short, these very old lines of descent separated early from all other lines and have remained separate from other lines for almost the entire known history of animal life on the planet. For my argument, this simply means that not all worms are the same thing, not even close. You are more closely related to a goldfish (both Chordates) than an earthworm (Phylum Annelida) is to a nematode worm (Phylum Nematoda).
We may never know with complete confidence what the first animals were. Much evidence points to sponges, some to comb jellies and cnidarians, and some to creatures such as placozoans, a still surviving group of very small and simple animals. None of these groups are worm groups, so apparently worms of all sorts may have arisen from non-worm groups. There are some trace fossils that appear to be tiny fossilized burrows dating back into the Precambrian, and at least some of these burrows may have housed worms of some type, though the small soft-bodied worms did not themselves fossilize. At any rate, worms did appear very early in the history of animal life, and by the time of the Burgess Shale fauna (515 million years ago) several distinct worm phyla were present. Today, some worms can swim in open water, and some can crawl on land or aquatic bottoms, while a great many, such as the common earthworm, burrow in sediments or soil. If life has evolved on some distant planet, that planet most likely has liquid water, and where you find abundant water you would expect to find bottom muds, as in the great expanses of the abyssal plains in our ocean basins—a perfect worm habitat.
Here on Earth, worms evolved independently several times and have adapted to almost all environments. There are as yet no flying worms, but there are worms in the soil, worms in freshwater, worms in our oceans, and vast hordes of parasitic worms within their host animals and plants. Some worms have adapted to the hellish temperatures of undersea hydrothermal vents, while others live in methane ice deposits deep in the Gulf of Mexico. Some inhabit regular sea ice near the poles. While the wildly diverse insects make up the majority of Earth’s species, if it were not for those familiar six-legged creatures, worms of all types would make up around half of all animal species. To get on with my point in as brief a summary as possible, I will resort to a list:
- The segmented worms (Annelida) include earthworms, leeches, and a huge diversity of marine species. It is now known that Annelids are close relatives of Molluscs, though whether the ancestor of the phylum Annelida was wormlike is as yet unknown.
- The Nematode worms (Nematoda), perhaps the most diverse worm phylum and sister group to yet another phylum of worms, the Nematomorphans.
- The flatworms (Phylum Platyhelminthes), including free-living forms and diverse parasitic worms such as the flukes and tapeworms. Flatworms were once thought to be a group ancestral to many of the other animal phyla. Now they are viewed as a more recently derived group that may have simplified body plans in comparison to those of their ancestral line.
- The ribbon worms (Phylum Nemertea), another distinctly different and ancient phylum of marine worms.
- The marine Parapulid worms (Phylum Parapulida), perhaps the least changed worms known, with fossil parapulids from the famed Burgess Shale strongly resembling some modern species.
- The velvet worms (Phylum Onychophora). This once marine group is today represented by a small number of tropical terrestrial species.
- The Rhombozoans, a separate and unique group of marine worm forms that are parasites of cephalopods, such as the octopus and squid. Notably, these tiny worms are the simplest known animals, with the fewest cells and the fewest cell types of any other kind of animal. They may well have simplified their body plan in adapting to parasitism from a once free-living ancestor.
- The acorn worms (Phylum Hemichordata). These marine worms make up a portion of their phylum that is shared by strange little animals called pterobranchs. They too appear to represent a separate origin of the worm form.
There are several more animal phyla containing worms, but I need to move on to another point. In addition to the many separate phyla of worms, there are several other animal phyla within which worm forms have evolved. Acanthocephalans are parasitic worms once believed to be a separate phylum but now known to be derived from rotifers, which are not worms (Phylum Rotifera)—so again, a separate evolution of worms. Pentastomids are parasitic worms also once thought to be a separate phylum but now known to be a type of Crustacean (Phylum Arthropoda) highly modified and changed from the typical crustacean morphology. Several insect groups (also Arthropods) have evolved complex life cycles that include body forms that could at least be construed as wormlike—the caterpillars, grubs, and maggots for example. There is one group of bivalves (phylum Mollusca) known as the shipworms that burrow into wood, and they at least look wormlike—thus the name. All the general facts about worm groups stated to this point can be found in Nielsen (2012) and/or Brusca et al. (2016).
There is one group of snakes called the worm snakes that have evolved to look much like earthworms, except that they have tiny eyes and jaws. Snakes themselves roughly resemble worms, and they evolved from lizards. As well, several other lizard groups have lost their legs (including glass lizards and some skinks) and evolved into elongate, legless forms. A group of amphibians known as the Caecilians have likewise lost their legs and evolved into forms resembling giant earthworms, complete with a burrowing lifestyle. In the marine phylum Echinodermata, a few sea cucumbers have become greatly elongated and undulate across the bottom in a wormlike fashion. Though they do have short legs, millipedes much resemble worms. Small eels resemble worms. The slugs and some nudibranchs of the phylum Mollusca at least come close to wormlike in form.
In short, the worm form body has evolved a great many times here on Earth, with many of the major worm groups having been around since the dawn of the animal kingdom. Collectively worms have diversified into at least a few million species. The worm form appears to be well adapted to many of the planet’s environments and can display almost every available lifestyle, including predators, herbivores, scavengers, parasites, filter feeders, etc. Though they are not all the same kind of animal, “worms” have collectively been hugely successful on this planet.
This evidence from our planet shows that worms have arisen independently many times. This evolutionary convergence on the worm form from many different ancestral groups at least suggests that worms might be a likely lifeform on other life-bearing planets. Simon Conway Morris has tried to use convergence as his main support of the idea that humanoids would be likely on other planets (Morris 2003). Without going into them, I find his arguments for the inevitability of humanoids to be fatally flawed. Much of his book consists of good examples of true evolutionary convergence, but he then strangely argues from convergence that sentient humanlike creatures should arise on other planets. He is mistaken to think that this assumption can be argued from convergence because humans are not convergent with any other species on Earth in either form or intelligence. We represent a single origin, while worms represent a great many separate origins. Humans are vertebrates, and the vertebrate group had a single origin in evolution within a single phylum of animals. Humans are mammals, then primates, and apes, and finally humanoids, and each of these increasingly specific groups had but a single evolutionary origin (that need not have occurred). There is simply no valid argument from convergence for the proposition that life on other planets will evolve humanoids with our level of intelligence. As others have pointed out, and as is clear from the wording in his book, Morris is undoubtedly biased in his argument due to his religious beliefs and his belief that there is purpose and meaning in the universe—including our own evolution.
To sum up, none of our pertinent evolutionary ancestry illustrates convergence of two or more groups or lines—rather our ancestry is one of consecutive single origins. Collectively, our planet’s “worms” have converged on their body form from many distinct ancestral groups, some of which we know were not wormlike. This does suggest, if we are to use evolutionary convergence as a guide, that at least some of those possible aliens out there might indeed resemble worms, and I doubt that worms could ever build a flying saucer or develop the technology to send and receive radio waves. Those who continue to hold that intelligent humanoids are a good bet for alien body forms simply don’t understand evolution very well and certainly don’t understand the vastness of the diversity of species that have lived, and that now live, on our home planet. If they did, they would clearly recognize that we humans are a single and unlikely latecomer, and our mere presence cannot be taken as evidence to support an argument for similar forms on other planets.
- Brusca, R.C., W. Moore, and S.M. Shuster. 2016. Invertebrates (3rd Edition). Sunderland, MA: Sinauer Associates, Inc.
- Morris, Simon Conway. 2003. Life’s Solution: Inevitable Humans in a Lonely Universe. Cambridge, MA: Cambridge University Press.
- Nielsen, C. 2012. Animal Evolution (3rd Edition). Oxford: Oxford University Press.
- Zeigler, D. 2008. Predicting evolution: How likely is it that human-level intelligence will evolve again? Skeptic 14(2)