EVOLUTION × LIFE

The Line That Leads to Us

~315 to 300 million years ago

Phanerozoic · Paleozoic · Carboniferous

Soon after vertebrates could live away from water for good, they split in two: one line towards reptiles and birds, the other towards 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

Measured across the whole Carboniferous and early Permian, our side rebuilt the body behind the head and grew large early; the reptile side left the body alone and reworked the temples and jaw joint over and over. Both had an early burst of change that slowed afterwards, and the reptile side stayed inside a narrower range of forms throughout. Much of what separates a mouse from a lizard was set going in those first few million years.

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