Question

Question #53742

Answer 1

Answer is vertebral column.

Explanation:

• The illustration that comes with the question presents evolution of vertebrates in very simplified order.
• It begins with hagfish, a jaw less vertebrate (Agnatha) related to evolutionary ancestors of vertebrates with jaws.
• First jawed vertebrates (Gnathostomata) were members of aquatic extinct group Placodermi. The jawed vertebrates are primarily of two types: superclass Pisces and superclass Tetrapoda.(Relatives of lung fishes acted as connecting link between the two.)
• Four classes under Tetrapoda are Amphibia, Reptilia, Aves and Mammalia. Human and his ancestors are included in order Primata.

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Now when we place another node in the beginning, it will represent a branching towards early chordate. Here an organism with permanent notochord in its body has been selected: e.g. Lancelet.

Evolutionary picture suggests that, a line of early chordates must have given rise to early jaw less vertebrates like Ostracodermi, another extinct group of aquatic organisms.