Our understanding of early animals was greatly increased with the discovery of the Burgess Shale in the early 20th Century. This site documents the "Cambrian explosion" of multicellular animal life about 500 million years ago, which has been attributed to a significant rise in soluble ocean oxygen, thought to have made thick and organized body plans possible for the first time. What made the Burgess site different was its exquisite preservation of soft tissues, revealing a great biodiversity of 'soft' animal life that lacked the 'hard parts' present in other animals, the latter of which are more easily fossilized and appear in similar numbers in all Cambrian sites around the world, including the Burgess.
Of particular interest was a group of partially soft-bodied animals called lobopods, which were segmented bilateral worm-like creatures with lobe-like legs. Evidence suggests that they walked, sensed, and perhaps most importantly, preyed on other less mobile animals. As such, they represent an important early step in a predator-and-prey arms race that drove the development of sophisticated sensorimotor functions.
There is a phylum of living lobopods called velvet worms, caterpillar-like creatures that live in moist environments like soil and rotting tree trunks. They bear a striking resemblance to some of their oldest fossil ancestors, suggesting that these early adaptations remain ecologically relevant. Velvet worms are thought to be a well-preserved descendant of a prototypical animal from which arthropods (including insects, spiders, and crustaceans) were derived, as they share many important body-plan characteristics.
Velvet worms don't look like our ancestors, however. The difference between the lineage leading to humans and that leading to velvet worms diverged very early on in the Cambrian: humans have one kind of embryonic development shared with starfish and sea urchins, while velvet worms share their embryonic development with squid and 'bugs'.
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