Box 2 Early evolution of the land plant life cycle
Land-plant life cycles are characterized by alternating multicellular
sexual (haploid gametophyte, n) and asexual phases (diploid sporophyte,
2n). Phylogenetic studies indicate that land plants inherited a
multicellular gametophyte from their algal ancestors but that the
sporophyte evolved during the transition to the land. Most megafossils
are sporophytes, and until recently there was no direct early fossil
evidence for the gametophyte phase. Recent discoveries of gametophytes
in the Rhynie Chert (Early Devonian, 380–408 Myr) have shed
new light on the evolution of land–plant life cycles18,20.