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Project Progress

Several studies were completed during Year 4:

Colonization of land by eukaryotes. We timed the colonization of land by fungi and plants with >100 nuclear proteins (Heckman et al., 2001). The origins of most major lineages were placed deep in the Precambrian, 1.4-0.7 Ga, considerably earlier than their fossils. Because fungi can enhance weathering, which in turn can lead to lower CO₂ levels and global temperatures, and land plants can bury carbon and generate oxygen, we proposed that this may have contributed to lower global temperatures and an increase in oxygen in the Neoproterozoic.

Earliest events. Many genes were transferred horizontally during or after the symbiotic events in eukaryote evolution, permitting the timing of those events with molecular clocks. With genomic data, an early split (~4 billion years ago, Ga) was estimated for archaebacteria and the archaebacterial genes in eukaryotes and at least two gene transfer events were identified in the origin of eukaryotes, at 2.7 Ga (premitochondrial) and 1.8 Ga (mitochondrial). Estimates for the origin of cyanobacteria (2.6 Ga) and the divergence of an early-branching eukaryote that lacks mitochondria (Giardia) (2.2 Ga) fell between those two events (Hedges et al., 2001). Those times have implications for early Earth atmosphere (oxygen) and the origin of eukaryotes.

Position of nematodes. The phylum Nematoda is believed to be one of the largest groups of animals, and it includes extremophiles. Molecular clock estimates have placed its origin at ~1.2 Ga. However, its phylogenetic position is highly debated, with most researchers supporting an alliance with arthropods (e.g., insects). We analyzed more than 100 genes, ordered by rate of evolution, and instead found significant support for a basal position with respect to arthropods and vertebrates, a finding that agrees with classical theories of animal evolution (Blair et al., 2002).