Most biologists agree that animals evolved from simpler single-celled organisms. Exactly how this happened is unclear, because few fossils have been left to record the sequence of events. Faced with this lack of fossil evidence, researchers have attempted to piece together animal origins by examining the single-celled organisms alive today.
The purple and yellow tube sponge displays one of the many different body forms typical of sponges. Sponges, considered to be the most primitive of the multicellular animals, are represented in the fossil record back to the Cambrian Period, at least 600 million years ago. The interior body cavities of sponges provide shelter for a variety of small crabs, sea stars, and other marine invertebrates.
Modern single-celled organisms are classified into two kingdoms: the prokaryotes and protists. Prokaryotes, which include bacteria, are very simple organisms, and lack many of the features seen in animal cells. Protists, on the other hand, are more complex, and their cells contain all the specialized structures, or organelles, found in the cells of animals. One protist group, the choanoflagellates or collar flagellates, contains organisms that bear a striking resemblance to cells that are found in sponges. Most choanoflagellates live on their own, but significantly, some form permanent groups or colonies.
This tendency to form colonies is widely believed to have been an important stepping stone on the path to animal life. The next step in evolution would have involved a transition from colonies of independent cells to colonies containing specialized cells that were dependent on each other for survival. Once this development had occurred, such colonies would have effectively become single organisms. Increasing specialization among groups of cells could then have created tissues, triggering the long and complex evolution of animal bodies.
This conjectural sequence of events probably occurred along several parallel paths. One path led to the sponges, which retain a collection of primitive features that sets them apart from all animals. Another path led to two major subdivisions of the animal kingdom: the protostomes, which include arthropods, annelid worms, mollusks, and cnidarians; and the deuterostomes, which include echinoderms and chordates. Protostomes and deuterostomes differ fundamentally in the way they develop as embryos, strongly suggesting that they split from each other a long time ago.
Animal life first appeared perhaps a billion years ago, but for a long time after this, the fossil record remains almost blank. Fossils exist that seem to show burrows and other indirect evidence for animal life, but the first direct evidence of animals themselves appears about 650 million years ago, toward the end of the Precambrian period. At this time, the animal kingdom stood on the threshold of a great explosion in diversity. By the end of the Cambrian Period, 150 million years later, all of the main types of animal life existing today had become established.