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.

It is however, quite unclear how this material could be linked to the pre-Chicxulub mass extinction. Possibilities of dust veiling to block out the sun and of the exotic aminoacids beiong highly poisonous have been considered. The sediment data shows Aib to be comparable in abundance to the ordinary aminoacids in the same samples. Aib also tracks these aminoacids rather than iridium levels. Microbial degradation processes as exist at the present epoch imply that Aib was really abundant at the time. Yet Aib is now known only within rare biological peptides, which are associated with select microfungi. Aib peptides generally have anti-biotic properties, derived from their structural propensity to form helical ion channels.

I hypothesise that Aib is an indicator of an exotic biology - probably with abundant Aib-fungi - that flourished through the K/T boundary. This fits with the concept that the genes coding for enzymes generating Aib-peptides arrived with microorganisms from space - perhaps as actual Aib-fungi. Alternatively the novel genes were incorporated into existing microfungi. The novel species of Aib-fungi were pathogenic to many Cretaceous organisms and were particularly virulent because of the novel biochemical properties. Their stress on faunal and floral species caused not only mass extinction but also accelerated evolution and speciation; species that evolved defence mechanisms tended to survive the fungal attack. The competitive evolution of the ordinary and alien organisms resulted in resistant species (including mammals) winning through and symbiotic relationships developing. While the fall-off in Aib in the upper sediments indicates that the exotic biology lost out in the end, the novel genetic input of the fungal invaders may have stimulated the evolutionary upsurge of the early Tertiary epoch.

References

1. Fred Hoyle's Universe: Proceedings of a Conference Celebrating Fred Hoyle's Extraordinary Contributions to Science, Chandra Wickramasinghe, Geoffrey Burbidge and Jayant Narlikar, eds., Kluwer, 2003.
2. M K Wallis, "Cosmic Genes in the Cretaceous-Tertiary transition," p 587-592 v 285, Astrophysics and Space Science, 2003: presented at Fred Hoyle's Universe conference, Cardiff University, June 2002.