Question

Which trait is found in early embryos of both birds and humans, but then disappears from each during development?

Answer 1

Pharyngeal (branchial) slits—often called "gill slits"—appear in early embryos of birds and humans. They do not develop into gills in either; instead they are remodeled into structures of the head and neck (e.g., parts of the ear, jaw, thymus, parathyroid) and disappear as gill-like openings.