Are There Possible Traces of Catastrophic Space Climate Events in the Hominid Fossil Record

The role of cataclysmic events in the evolution of life on Earth has been discussed in recent times. Tobias has suggested the possible causal connection between large impacts such as the Vredefot impact structure in the Free State (it is the latest

World Heritage Site to be listed in South Africa). Its reconstructed diameter of 250-300 km was made by a projectile estimated at 10-15 km in diameter, which collided with the Earth at 2.1 Ga (Tobias, 2005).

This impact coincided with two significant events in the evolution of life on Earth, namely the oxygenation of the atmosphere and the first appearance of the eukaryotes. Although Tobias attempts to make a causal connection between the large impact and these two events, an approach that he calls catastrophism, there are alternative explanations as discussed by others: in the case of the oxygenation of the atmosphere (Abelson, 2007), and the first appearance of the eukaryotes (Chela-Flores, 1998). But even if the alternative explanations are maintained, what is true is that the Vredefot impact illustrates the major implications that extraterrestrial events, not only SpW, as illustrated in this paper, but even planetary sterilizations going back into the Phanerozoic and extending back to the Hadean.

Tobias insists that milder environmental impacts might have been relevant in the evolution that led to Homo sapiens. For instance, about 2.6-2.5 Ma marked climatic changes in Africa that were associated with uplift of its southern and eastern parts. The ensuing cooler and dryer weather was accompanied with significant changes in the paleontological record:

(a) Extinction of the small-brained hominids Australopithecus africanus

(b) The earliest appearance of Homo of the species Homo habilis

(c) The first signs of the enlargement of the hominid brain, as compared with the smaller brains of the australopithecines

The possibility of a supernova explosion near the Solar system has been discussed for a long time (Ruderman, 1974; Reid et al., 1978; Ellis and Schramm, 1995). Such a nearby supernova explosion can be confirmed by the detection of radioisotopes on Earth that were produced and ejected by the supernova. A measurement of a well-resolved time profile of the 60Fe concentration in a deep-sea ferromanganese crust showed a significant increase 2.8 Ma (Knie et al., 2004). The amount of 60Fe is compatible with the deposition of ejecta from a supernova at a distance of a few 10 pc. The well-defined time of the supernova explosion makes it possible to search for plausible correlations with other events in Earth's history. Other possible radionuclides for tracing supernova explosions are 182Hf (8.9 Ma) (Vockenhuber et al., 2004), 244Pu (81 Ma) (Winkler et al., 2004).

The profile of the 60Fe concentration in the deep-sea ferromanganese crust has been considered in terms of the environmental changes that were relevant for Homo evolution (a-c) According to the authors (Knie et al. 2004), at the time of the supernova explosion there was an increase of the cosmic radiation of a few percent that lasted for some thousand years. They claim this might have triggered climate change in Africa, causing significant developments on hominid evolution. This effect would in any case be superimposed on other phenomena causing climate change, such as tectonic activities (like those that gave rise to the Great Rift Valley in Africa), as well as other global phenomena.

Besides, this event could have a significant effect on the ozone layer. Hence it may have had an effect on the natural UV filter that led to the present Earth biota. Improved tools for detailed modelling of atmospheric chemistry have been developed to calculate ozone depletion, and advances have been made also in theoretical modelling of supernovae and of the resultant gamma-ray spectra. In addition, we now have better knowledge of the occurrence rate of supernovae in our galaxy and of the spatial distribution of progenitors to core-collapse supernovae. The results of two-dimensional atmospheric model calculations estimates (Gherls et al., 2003) are interesting in this respect, since they take as input the spectral energy distribution of a supernova, adopting various distances from Earth and various latitude impact angles. In separate simulations there is an estimate of the ozone depletion that is due to cosmic rays. These calculations suggest that for the combined ozone depletion from these effects roughly to double the "biologically active" UV flux received at the surface of the Earth, the supernova must occur at 8 pc. Based on the latest data, the time-averaged galactic rate of core-collapse supernovae occurring within 8 pc is 1.5 Ga.

In principle, high-energy galactic cosmic rays could be also responsible for genetic changes related to human evolution. Some groups have been searching for discrepancies in the production rate of stable cosmogenic radionuclides, such as 21Ne and radioisotopes with different half-lives such as 10Be, 26Al and 53Mn that might indicate time variation in the galactic cosmic-ray flux within the solar system. The existing data do not support major variations in cosmic ray intensity within the past 5 million years, crucial period for the evolution of the Homo species (Moniot et al., 1983). A huge asteroid collided with the Earth in South East Asia around 780,000 years ago, with devastating environmental effects. Magnetic properties of rocks formed at that time show that the impact of the extra-terrestrial body might have caused the Brunhes-Matuyama magnetic reversal. Tonnes of tektites - obsidian-like pebbles produced by the fusion of sediments during the impact - were launched into the air and scattered all over South-east Asia and Australia. Some scholars connect this environmental disaster to the introduction of advanced Acheulean-like technology in Asia during this period.

More recently it has been suggested that a comet or asteroid exploded over North America 13,000 years ago. This event wiped out a Stone Age culture known as Clovis, as well as the mammoth and the mastodon. This event may have caused a major shift in the climate, the well-known Youger Dryas cooling event. The cooling produced may also have affected humans in Europe and Asia (Firestone et al., 2007). A detailed analysis of the sediments corresponding to 13 ka ago reveal a high concentration of extraterrestrial (ET) markers such as glass-like beads, soot and fullerenes, materials that are absent in other layers of the stratigraphy. The glassy beads could only be produced by melting carbon at 4,000°C. Electron microscope analyses show the glassy spherules are reach in micro-diamonds. Diamonds are produced in the interior of the Earth by compressing carbon at the pressure of several gigapascals.

These conditions could be produced on the surface of the planet only by the impact of a massive extraterrestrial body.

Finally, we should mention that the evolution and dispersion of the Homo species during the last 2 million years was strongly conditioned by the variable climate of Earth, driven by changes in the Earth's orbit around the Sun as proposed by Milankovitch in the 1920s. Three variations of the Earth's orbit are considered, eccentricity, obliquity and precession, which affect the quantity of sunlight hitting the Earth's surface. They are the main cause of the ice ages during the Pleistocene, characterised by periods of about 100,000, 40,000 and 20,000 years, respectively, as confirmed by the ice fossil record in sea sediments.

0 0

Post a comment