Fossilized tracks from an early reptile are now the oldest known reptilian tracks, meaning the tetrapod ancestor most likely appeared earlier, during the Devonian period. Humans are tetrapods, and like all tetrapods (except amphibians), we are also amniotes, with eggs that protect developing embryos in amniotic sacs. Amniotes are thought to have diverged from amphibians at the dawn of the Carboniferous period, about 355 million years ago. They now think that the tetrapod ancestor appeared during the Devonian, and that amniotes began to diverge from them about 395 million years ago, 35 to 40 million years earlier than previously thought. “The [fossil footprints] have a disproportionate impact on our understanding of early tetrapod evolution because of their combination of diagnostic amniote characteristics and early, securely constrained date,” the researchers .