Episode 5 of The Story of Life on Earth covers roughly 485 to 359 million years ago, when nothing in the ocean could yet bite. Every mouth could only suck, filter, or scrape, until a set of bones built for breathing hinged forward and produced the sea’s first jaw.
The episode traces the gill arch hypothesis for how a jaw evolved from breathing bones, the rise of jawed placoderms like Dunkleosteus, the early shark Cladoselache, and the 2013 fossil discovery, Entelognathus, that reshaped the debate over where jaws came from. Every hinged jaw alive today, human beings included, traces back to that breakthrough.
Two extinction pulses near the end of the Devonian wiped out most of the placoderms, but the jaw itself did not disappear with them. It carried forward into sharks, bony fish, and eventually every land vertebrate that followed. The 2013 discovery of Entelognathus, a placoderm with a bony jaw structure closer to later fish than expected, pushed scientists to rethink which features came first, complicating a story that once looked much simpler.
It’s the same long story of deep time that runs through Martin’s 2019 book, Life: The First Four Billion Years, illustrated by Grahame Baker-Smith, which follows Earth’s history from the Big Bang through the fossil record and the age of dinosaurs to the first mammals.
Once a jaw existed, it changed what predation itself could look like. A mouth that could grip and shear meant animals could target prey too large or too armored for a suction feeder to manage, which put new pressure on everything else in the water to grow armor, speed, or size of its own. Paleontologists reconstruct this shift mostly from fossil skulls, comparing the arrangement of gill arch bones across placoderms, sharks, and early bony fish to work out which structures are ancestral and which evolved later, the same kind of comparative anatomy that made the Entelognathus find so disruptive when it didn’t fit the expected pattern.
Watch Episode 5 on YouTube, start from Episode 1, or see the full series on the YouTube Series page.