Late Precambrian bilaterians: grades and clades

Area of Science:

• Evolutionary biology
• Paleontology
• Developmental biology

Background:

• The Early Cambrian period witnessed an unprecedented diversification of animal body plans.
• Ancestral lineages preceding this explosion are primarily identified through trace fossils.

Purpose of the Study:

• To investigate the origins and drivers of the Cambrian explosion's morphological novelty.
• To reconstruct ancestral forms and understand the genetic underpinnings of early animal evolution.

Main Methods:

• Integration of fossil record data with molecular phylogenetic analyses.
• Modeling of cell-phenotype number increase to estimate metazoan origins.
• Analysis of Hox/HOM gene cluster presence in Cambrian phyla and their ancestors.

Main Results:

• The last common ancestors of major bilaterian groups were likely simple, worm-like organisms.
• Metazoan origins are estimated around 600 million years ago, with a subsequent passive increase in complexity.
• The Hox/HOM gene cluster was likely present in Precambrian trace-making ancestors.

Conclusions:

• The Cambrian explosion involved significant repatterning of gene expression for novel body plan development.
• This evolutionary event did not necessitate a sudden increase in genomic or morphological complexity.
• Early animal evolution was characterized by rapid diversification from relatively simple ancestral forms.