Planet of the Bugs: Evolution and the Rise of Insects by Scott Richard Shaw

diversity. Success might be measured in various ways. We are ever so proud of our oversized contemplative brains. But with those brains we must ponder the sheer improbability of our existence and our constant vulnerability through the ages. The humble insects cannot contemplate their own measure of success: the numerical dominance of arthropod species through all ages of animal life. If this were to all play out again on another planet, it seems to me highly improbable that soft-bodied creatures with internal skeletons would develop first or become successful over the long run. Hard-covered creatures with external skeletons would almost certainly hold the advantage over time, in most contests of soft versus hard-shelled players.
    Looking back on the earliest Cambrian trace fossils, those worm burrows, we know that even those simple animals could tunnel in sediments. We don’t suppose that they had any legs yet, because there are no fossil footprints. So how did they move? We must assume they had muscles arranged in body segments, allowing them to contract segments and wiggle their bodies, as with modern earthworms. Segmentation is a common body form, but it is an ancient one as well. All modern insects are segmented animals; hence the name “insect,” which means “in sections.” But that characteristic is not unique to insects. It is an inherited trait from earlier ancestors. All arthropods, including the trilobites, were segmented creatures, and so were simpler creatures, such as the annelid worms. That ancient burrowing creature
Trichophycus pedum
was probably also segmented, precisely because it could tunnel, but did not have apparent legs.
    The origins of segmentation clearly reside in the earliest multicellular animals. Just as single cells became multicellular aggregations by building duplicates of themselves, early multicellular creatures became segmental by building duplicates of their cellular arrangements and linking them in a chain. Segmentation is an excellent trait for an animal, not just because the components are easiest to build and link together, but also because the shock is less when you lose a part to an accident. If you have seen the science-fiction movie
The Core
, you mayrecall the following example: explorers used a multisegmented craft to travel deep into the earth, losing parts along the way but surviving. Trilobites and insects can lose body parts and survive more easily than we can.
    It’s no mistake that the earliest skeletal parts were on the outside. As I already mentioned, it makes more sense to shunt waste products to the exterior than to pile them up on the inside. If an ancient wormlike creature were evolving a hard outer skeleton, it is only logical that the skeleton would form in segmental plates, just as in all arthropods. They already had segments with muscles. Body flexibility could only be maintained if the segmental parts remained flexible, with membranes at the edges. Any attempt at a fully hardened exterior would be useless and maladaptive, because a completely hardened creature could not move at all.
    The other unique feature of arthropods, and another key to their success, is their multijointed legs. Take a moment to imagine a delicious plate of steaming Alaskan king crab legs, and you know what I’m talking about. Jointed legs in hard shells, that’s the feature that defines the arthropods and the characteristic was passed along to the insects. The name “arthropod” translates to “jointed foot.” Here’s an easy way to remember that. You recall that when you have problems with your joints, we call it arthritis. For problems with your foot, you see a podiatrist. So, arthropods have a jointed foot.
    Among the Cambrian fossils there are several indications of leg origins. Some Early Cambrian annelid worms like
Burgessochaeta
have short paired protuberances on each segment. These appendages were not jointed but had long bristles. Clearly they were not legs but could