A hypothetical primitive ancestor organism that resembles or had characteristics similar to a gastrula, proposed in 19th-century evolutionary theory.
From gastrula plus -an suffix suggesting 'like' or 'pertaining to.' Coined by Ernst Haeckel and other 19th-century naturalists theorizing about evolutionary ancestors.
Haeckel's 'gastruran theory' suggested all animals evolved from a common ancestor that looked like a gastrula—it's not quite right, but it brilliantly highlighted how universal the gastrula stage is across all animal life.
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