2002 Annual Science Report
Pennsylvania State University Reporting | JUL 2001 – JUN 2002
(S. Blair Hedges) Timescale for the Evolution of Life on Earth: Molecular Evolutionary Approach
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).
PROJECT MEMBERS:S. Blair Hedges
RELATED OBJECTIVES:Objective 2.0
Develop and test plausible pathways by which ancient counterparts of membrane systems, proteins and nucleic acids were synthesized from simpler precursors and assembled into protocells.
Expand and interpret the genomic database of a select group of key microorganisms in order to reveal the history and dynamics of evolution.
Describe the sequences of causes and effects associated with the development of Earth's early biosphere and the global environment.
Identify the environmental limits for life by examining biological adaptations to extremes in environmental conditions.
Search for evidence of ancient climates, extinct life and potential habitats for extant life on Mars.
Determine (theoretically and empirically) the ultimate outcome of the planet-forming process around other stars, especially the habitable ones.
Define climatological and geological effects upon the limits of habitable zones around the Sun and other stars to help define the frequency of habitable planets in the universe.
Determine the resilience of local and global ecosystems through their response to natural and human-induced disturbances.