Life › Complex cells & first multicellular life › Ediacaran biota
Bilaterian animals: bodies with front & back
by c. 555 million years ago (Ikaria)· Evolutionary innovation· Animals
Strong evidenceGood evidence; dates or details still being refined.
Animals with a front and back, left and right sides, and usually a gut running from mouth to anus.
See it on the Life map →What happened
Bilaterians include worms, insects, molluscs, starfish and vertebrates: nearly all animal species. Most have three tissue layers, a head end that leads, and a through-gut. One of the oldest known is Ikaria wariootia from Nilpena, South Australia, about 555 million years old: a worm-like animal the size of a rice grain, identified in 2020 with 3D laser scans and linked to nearby burrows. Molecular clocks suggest the group is older.
Why it matters
A body with a leading head and a through-gut favors directed movement, burrowing and hunting, the lifestyles that drove the Cambrian explosion.
The evidence (fossils, genes, rocks)
Fossil burrows from about 560 million years ago, Ikaria and Kimberella body fossils, gene trees, and shared developmental genes such as the Hox clusters.
Still debated
What the common ancestor (the 'urbilaterian') looked like, a simple flatworm-like crawler or a more complex segmented animal, is debated.
Inside this (1)
Connections (2)
▸Hox genes: the body-plan toolkit pattern the bodies of Bilaterian animals: bodies with front & backWell established
The connection
Hox genes label positions along the head-to-tail axis, telling each segment or region what to become. Bilaterians, with their distinct front and back, all use versions of the same set: a fly's head and a mouse's neck are patterned by related Hox genes inherited from a common ancestor.
When and how
In the last common ancestor of bilaterians, before about 555 million years ago; discovered through fruit fly genetics, with the shared DNA motif found in 1983–84.
The evidence
Hox clusters with matching gene order occur in insects, worms and vertebrates, and related genes switch on in the same head-to-tail order in fly and mouse embryos.
Debates and open questions
How many Hox genes the first bilaterian had, and how jellyfish and sea anemones, which have related genes, use them.
▸Bilaterian animals: bodies with front & back diversified in Cambrian explosionWell established
The connection
Most of the explosion was bilaterian: arthropods, molluscs, worms, brachiopods, echinoderms and chordates. With a head, a through-gut and directed movement, bilaterians could burrow, hunt and graze in new ways. Small Ediacaran bilaterians like Ikaria were forerunners of this burst.
When and how
From about 555 million years ago (Ikaria) to about 515 million; key sites include Chengjiang, China, and the Burgess Shale, Canada.
The evidence
Most Cambrian fossils belong to living bilaterian groups, and molecular trees place their splits before or during the Cambrian.
Debates and open questions
Why bilaterians stayed small and rare for millions of years before suddenly diversifying.