In evolutionary biology, a hypothetical ancestral organism with a two-layered body structure, believed to be the common ancestor of most animals.
From Greek 'gaster' (belly). Coined by German biologist Ernst Haeckel in the 19th century as part of his 'Gastraea Theory,' which proposed this as the primitive form of multicellular life.
Haeckel's gastraea theory is largely outdated by modern genetics, but it was revolutionary because it suggested that if you study how embryos develop (a process called gastrulation), you can see echoes of how your ancient ancestors looked!
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