Walter Garstang - Garstang's Hypothesis

Garstang's Hypothesis

Walter Garstang noted from study of marine animals that both echinoderms and chordates are deuterostomes (in which the blastopore forms the anus), while most other possible ancestors of Chordates are protostomes (the blastopore forms the mouth). This inspired Garstang to suggest an alternate route of evolution: from echinoderms to chordates.

There are many important differences between chordates and echinoderms. Most importantly, adult echinoderms show little likeness to chordates: echinoderms are radially symmetric, possess calcium carbonate plates in their skin and have tube feet. Garstang made the radical suggestion that perhaps it was echinoderm larvae, not adults, which had given rise to chordates.

Echinoderm larvae, like chordates, are bilaterally symmetric. Especially notable are their similarities to larvae of hemichordates, which are a step closer to chordates as they share two of the five most commonly noted chordate characteristics, namely a hollow neural tube and pharyngeal slits.

Garstang's idea has been expanded and is supported by many lines of evidence. Perhaps most interesting and compelling is the fact that some amphibians can stay in larval form and still reach sexual maturity—this shows that echinoderm larvae could, theoretically, have become sexually mature and simply stopped morphing into adults, instead evolving into chordate ancestors. Species that show this refusal to leave the larval stage include mud puppies and other salamanders, which either partially or completely show neoteny (also called pedomorphism): retention of juvenile traits or phenotypes after sexual maturity.

Garstang's Hypothesis was revolutionary for both its time and idea: it suggests that not only may single species evolve, but that single life stages of species may evolve into separate organisms. The hypothesis, which Garstang proposed in the early 20th century, seemed far-fetched at the time of its conception and did not receive support until after Garstang's death.