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Swalla studies the evolution of chordates, the phylum that includes both vertebrates and tunicates.

During the Cambrian, which began about 540 million years ago, nearly all modern animal groups—as diverse as mollusks and chordates—leapt into the fossil record.

Some of the problems that once stalled progress — such as uncertainty over which groups belong in the deuterostome lineage of animals alongside chordates — have largely been resolved.

Having established a hazy picture of the earliest chordates, Gee focuses on building vertebrates and their defining features from the basic chordate body plan, for example through spectacular innovations in the vertebrate head.

The morphological divide between chordates and their closest relatives remains perilously large, and even with new advances, Gee’s hypothesis is a good contender; but it is not the only one, so the bridge to early chordates remains a little wobbly.