It’s possible to make a pretty good stab at where most animals sit on the tree of life. It doesn’t take a huge imaginative leap to see how fish could give rise to amphibians, reptiles, birds and mammals, or how a worm-like creature could, with a few tweaks, develop into something like an insect or a mollusc.
But where the heck does a starfish fit into the evolutionary scheme of things? With five limbs sporting hundreds of hydraulic rubbery feet, no head, or even a front, rear, left or right, it seems to bear little resemblance to the rest of life on Earth. The same goes for its relatives: sea urchins, brittlestars, sea cucumbers and featherstars could all have arrived here from another planet or a parallel universe.
What are echinoderms?
Collectively, these other-worldly invertebrates are members of the phylum Echinodermata, a name that translates from the Greek as ‘spiny skinned’. Few animals are as spiny as a sea urchin. But echinoderms are also dotted with characteristic projections called pedicillariae – tiny, movable structures, some of which look like miniature grasping claws, which have a variety of functions including defence, feeding, sense, and removing debris and encrusting organisms from the skin.
There are about 7,000 echinoderm species, every one of which lives in the ocean, from intertidal rockpools to the darkest depths. Most are filter-feeders, using their limbs to sieve food from the water or sediment. The limbless sea urchins tend to graze food growing on the surfaces of rocks, and starfish are largely scavenging generalists, though some are active predators of molluscs and other echinoderms.
Perhaps echinoderms’ most striking peculiarity is their body plan. Most animals are bilaterally symmetrical, with left and right sides that are more or less mirror images of each other. But echinoderms are radially symmetrical, with five planes of symmetry passing through a centre point, like a pentagon. This is most obvious in the starfish and brittlestars, most of which have five arms, or multiples thereof. Intriguingly, their free-swimming larvae are bilaterally symmetrical, but as they develop their left side grows at the expense of the right, eventually engulfing it entirely.
As for where echinoderms fit on the evolutionary tree, they occupy a branch of their own that first sprouted at least 500 million years ago. At its base, it joins one of two larger branches, which represent the two major groups of complex animals, distinguished on the basis of how their early embryos develop. One, the protostomes, includes the vast majority of invertebrates – arthropods, molluscs and worms. The other, the deuterostomes, includes little more than the echinoderms and the chordates, the lineage that led to the vertebrates. In which case, we humans are among echinoderms’ closest living relatives. It’s just that after half a billion years of independent evolution, it’s hard to see the family resemblance.