When did terrestrial plants arise?

Microfossils suggest that co-option of algal genes may affect land plant origination time Embryophytes—or land plants—are fundamental to life on Earth and influence systems that include carbon cycling, sedimentation rates, and rock weathering ( 1 , 2 ). Although much is known about their probable mid-Ordovician appearance [about 450 million years ago (Ma)] ( 3 ) and later diversification on the basis of fossils from late Silurian and Early Devonian periods (about 420 Ma) ( 4 , 5 ), the time of their origination has been unclear. This knowledge is important when considering geological processes and evolutionary interactions. There has been a discrepancy in the time of land plant origination between molecular clock estimations (based on genes and RNA) and fossil record estimates (based on morphology). On page 792 of this issue, Strother and Foster ( 6 ) describe fossilized spores whose characteristics raise the possibility that land plants arose by co-opting algal genes, along with acquiring de novo genes, and that the former would account for the molecular clock predating the fossil record.