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    'Cosmic pathogen' may have doomed dinosaurs


    Is the exotic amino acid (AIB) in sediments below and above the K/T boundary the mark of a cosmic pathogen? Such a novel cause of the downfall of the dinosaurs is appropriately hypothesised in the new book, Fred Hoyle's Universe (1), and in a paper by Max Wallis (2), of which the following is a condensation.

    Summary - It is proposed that genes coding for Aib-polypeptides arose early on in the K/T transition, presumed from the Earth's accretion of interplanetary (comet) dust. Aib-fungi flourished because of the evolutionary advantage of novel antibiotics. The stress on Cretaceous biology led directly and indirectly to mass species extinctions, including many dinosaur species, in the epoch preceding the Chicxulub impact.

    If cosmic genes are to be a driver of species evolution, Hoyle and Wickramasinghe developed the idea of pathogenic carriers to establish them in terrestrial ecosystems. Carriers could be viruses or more complex organisms. That viable bacteria could reach the Earth within meteorites, such as those known to derive from Mars, is now considered fully plausible. But such material has been reaching the Earth throughout evolutionary history, so it would take material from more unusual sources to cause significant episodic evolutionary changes.

    Consider the major evolutionary change of the Cretaceous-Tertiary (K/T) transition. The K/T boundary as measured by the spike in iridium abundance is now well identified with the asteroid or comet impact that caused the Chicxulub crater. However, the K/T transition as measured by the mass extinctions of Cretaceous species lasted of order 100 kyr or more, right across the 65.0 Myr impact event. Global wild-fires and atmospheric changes consequent on the impact cannot explain the earlier extinction record.

    The iridium data do provide an extraterrestrial signal pre-Chicxulub impact. The iridium abundances in samples above and below the impact spike in the boundary sediments at Stevns Klint (Zhau and Bada 1989) indicate a strong enhancement of extraterrestrial debris reaching the Earth over some 100 kyr. The same boundary clays are also remarkable for high levels of the exotic aminoacid Aib, which was known from the 1980's to be a major aminoacid in extracts from the Murchison meteorite. By "exotic" I mean a non-protein aminoacid, not in the standard 21 or 22 making up DNA. Zhau and Bada therefore hypothesised (1989, Nature 339, 463) that Aib and a second exotic aminoacid, isovaline, are remnants of extraterrestrial organic material.

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