2011 Annual Science Report
University of Wisconsin Reporting | SEP 2010 – AUG 2011
Project 1D: Evolution of Life Related to the Development of the Earth’s Core
We investigated the effect of evolution of the Earth’s core on the magnetic field, which bears on the extent of magnetic shielding of cosmic radiation, which in turn has implications for the evolution of life. We hypothesize that extensive magnetic shielding likely occurred only after turbulent flow in the liquid outer core subsided, which would have occurred after 1 or 2 b.y. of inner core solidification, allowing differential rotation of the inner and outer core. These results suggest that life which evolved in the first 1 to 2 b.y. of Earth history would have had to develop strategies to cope with high cosmic radiation. The most effective strategy is to use shielding by absorption of radiation by water. Consequently evolution of life on the surface of the Earth occurred much later in the Earth’s history.
We have sought to relate biological evolutionary history with the physical evolution of the Earth. We do not know when life originated on Earth but by 3.45 Gyr ago well-preserved stromatolites attest to the likely existence of (not necessarily oxygenic) photosynthetic bacteria, established in submarine environments. From that time although there were evolutionary advances, the major diversification started more than 2 billion years later and even more complex life only after 3 billion years.
Whereas biomarkers, molecular data, and body and trace fossils have been interpreted often as an indication of a much earlier origin for putative eukaryotes, recent models integrating well-constrained fossil dates and robust molecular data posit that the early diversification of extant basal eukaryotes did not occur earlier than 1.2-1.1 Gy. Use of the microfossil record to calibrate eukaryote phylogeny, it has been estimated that the basal radiation of extant eukaryotes occurred near the Meso-Neoproterozoic boundary, approximately 1.1 Gy (but in the range 0.95–1.35 Gy). In this interpretation, this basal radiation was shortly followed by the divergence of amoebozoans, opisthokonts and bikonts; also all other major eukaryotic supergroups radiated in the Neoproterozoic. Therefore, although their common root may be earlier, all major basal lineages of eukaryotes arose at about this time. In addition, the earliest fossils of putative non-marine eukaryotes are recorded in this timespan, between 1.2–1.0 Gy. This is consistent with the idea that early eukaryotes should be associated with environments once exposed on the Earth’s surface. We have sought to relate this evolutionary history with the physical evolution of the Earth.
The early Earth was mostly undifferentiated, being a hot aggregate of planetesimals. By mantle convection, downward limbs of convection cells would leave some of their denser constituents at the base of the mantle while less dense components rose to form the crust. The heavy elements, in particular Fe and Ni, sank to form the core, where the higher temperature maintained its liquid state. Irreversible mass redistribution within the core is controlled primarily by inner core growth, which has been calculated to occur at rates between 0.2 mm/yr and 0.7 mm/yr. However, energy conservation, when applied to the Earth’s core and integrated between the onset of the crystallization of the inner core and the present time, leads to an estimate of the age of the inner core. This age is a function of the heat flux at the core-mantle boundary and the concentrations of radioactive elements. The age estimate for the inner core varies between 0.5 to 2.5 Gyr ago with a most likely value of 1 Gyr. We assume that the rotation of the inner core relative to the outer core. Before the existence of the inner core we expect a lower, or at least different, magnetic field. Before the solidification of the inner core, the faster relative rotation of it relative to the external liquid core was obviously absent. This novel development in Earth’s dynamics may have triggered a stronger dipolar component of the magnetic field.
Prior to the development of the stronger magnetic field, which protected the Earth from intense radiation, it is clear that life had evolved considerably and was abundant. This was possible only because of the radiation absorption powers of water allowing subaqueous microbial life to flourish. The main part of the radiation flux consists of electrons and protons from the solar wind, but also includes the most penetrating radiation, relatively heavy ions. The depth of penetration in water or ice has been calculated and the intensity is attenuated by at least 6 orders of magnitude for even heavy ions in about one meter depth of water.
This would have allowed microbial evolution to occur under water while the Earth’s core and magnetic field protection developed and at that stage near-surface non-marine life also became possible. Moreover, although generally interpreted as life-limiting, Snowball Earth conditions with snow and ice covers on the oceans may have favored radiation protection. Therefore we hypothesize that the development of life on Earth was significantly affected by the growth of the solid inner core.