EVOLUTION × LIFE

The Line That Leads to Us

~315 to 300 million years ago

Phanerozoic · Paleozoic · Carboniferous

Vertebrates won their independence from water, and almost at once the new group split in two: one line leading to reptiles and birds, the other to mammals. Early members of both looked alike, low and lizard-shaped, though some on our side carried a sail of tall spines down the back. What separated them was not appearance but where each one experimented.

Why it matters

Across the whole Carboniferous and early Permian, our side rebuilt the body behind the head and reached large size early; the reptile side changed that body far less and worked instead on the temple region, and with it the mechanics of the skull and jaw. Both had an early burst of change that slowed afterwards, and the reptile side stayed within a narrower range of forms throughout. Much of what separates a mouse from a lizard was already set going by that first divergence.

Dating & uncertainty

The amniote root is placed near 320 Ma and the split between the mammal and reptile stem lineages follows closely, but both dates rest on a thin Carboniferous fossil record and shift with each phylogenetic reanalysis.

Sources

1. Brocklehurst et al. (2022), Systematic Biology · Early origins of divergent patterns of morphological evolution on the mammal and reptile stem-lineages
2. Brikiatis (2020), Scientific Reports · An early Pangaean vicariance model for synapsid evolution
3. Matamales-Andreu et al. (2024), Nature Communications · Early-middle Permian Mediterranean gorgonopsian suggests an equatorial origin of therapsids

Earth history as one day

22:20:051 h 40 min before midnight

if Earth’s 4.54 billion years were compressed into a single day