Some paleontologists suggest that animals appeared much earlier than the Cambrian explosion, possibly
as early as 1 billion years ago. Trace fossils such as tracks and burrows found in the Tonian era indicate
the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex
as earthworms. During the beginning of the Tonian period around 1 billion years ago, there was a
decrease inStromatolite diversity, which may indicate the appearance of grazing animals, since
stromatolite diversity increased when grazing animals went extinct at the End Permianand End
Ordovician extinction events, and decreased shortly after the grazer populations recovered. However the
discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled
protist Gromia sphaerica casts doubt on their interpretation as evidence of early animal evolution.
It has been estimated that 99.9% of animals that have ever existed are extinct.
Phylogenetic analysis suggests that the Porifera and Ctenophoradiverged before a clade that gave rise to
the Bilateria, Cnidaria andPlacozoa. Another study based on the presence/absence ofintrons suggests
that Cnidaria, Porifera and Placozoa may be a sister group of Bilateria and Ctenophora.
The sponges (Porifera) were long thought to have diverged from other animals early. They lack the
complex organization found in most other phyla. Their cells are differentiated, but in most cases not
organized into distinct tissues. Sponges typically feed by drawing in water through pores. Archaeocyatha,
which have fused skeletons, may represent sponges or a separate phylum. However, a phylogenomic
study in 2008 of 150 genes in 29 animals across 21 phyla revealed that it is the Ctenophora or comb
jellies which are the basal lineage of animals, at least among those 21 phyla. The authors speculate that
sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be
Among the other phyla, the Ctenophora and the Cnidaria, which includes sea anemones, corals,
and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as
both the mouth and the anus. Both have distinct tissues, but they are not organized into organs. There
are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As
such, these animals are sometimes called diploblastic. The tiny placozoansare similar, but they do not
have a permanent digestive chamber.
The remaining animals form a monophyletic group called the Bilateria. For the most part, they
are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body
istriploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive
chamber has two openings, a mouth and an anus, and there is also an internal body cavity called
a coelom or pseudocoelom. There are exceptions to each of these characteristics, however — for
instance adult echinoderms are ra