Comets and the Origin and Evolution of Life

2 J. Oró, A. Lazcano, and P. Ehrenfreund
suggest that many different types of cyclic and acyclic organic compounds
may be present in cometary nuclei (Kissel and Krueger, 1987); (d) the possibility
that the Cretaceous–Tertiary (K/T) extinction was caused by a large
meteorite (Alvarez et al., 1980) or a comet (Davis et al., 1984); and (e) the
detection of extraterrestrial nonproteinic amino acids near the K/T 65 million
year old boundary, which may be of ultimate cometary origin (Zhao and
Bada, 1989; Zahnle and Grinspoon, 1990).
Since the reconstruction of the physical and chemical characteristics of
the primitive environment is fraught with problems and pitfalls, it is difficult
to evaluate in detail the cometary contribution to prebiological processes.
This has led to rather extreme points of view, which range from claims that
bacteria and viruses originating in the interstellar medium pass from comets
to the Earth (Hoyle and Wickramasinghe, 1984; Ponce de Leon and Lazcano,
2003) to a sobering challenge of the idea that the input of extraterrestrial
organic molecules and volatiles played a major part in the origin of terrestrial
life (Miller, 1991a, b). As argued throughout this volume, the role that comets
may have played in prebiotic and biological evolution probably is important.
As summarized in this introductory chapter, a widely held idea supported
by different independent estimates is that cometary impacts represented a
rich veneer of volatiles during the early stages of the Earth’s history. As the
solar system aged, such catastrophic events became increasingly rare, but
nonetheless they may have had dramatic implications for the evolution of
the biosphere (Steel, 1997), and may represent a potential threat for our own
species (Morrison, 1997). The purpose of this chapter is to review the different
(and sometimes conflicting) hypothesis on the role of comets in the origin and
evolution of life, and to provide a brief summary of the historical background
that led to the development of these different ideas.
1.2 Comets and the Origin on Life: An Idea
with a Long History
There is an old scientific tradition linking cometary phenomena with the appearance
of life. Prompted and funded by Edmund Halley, Isaac Newton published
in 1686 his Principia, a book that represents a major breakthrough in
the development of Western thought. But not even the grave and pompous Sir
Isaac, for all his love for mathematical accuracy, was immune to the appeal of
popular beliefs, even if they had not been experimentally proven. As summarized
by Oparin (1938), more than once Newton expressed his conviction that
cometary emanations could lead to the spontaneous generation of plants – a
phenomenon neither he nor Halley had the chance to corroborate when the
latter’s comet returned to perihelion in 1795.
A hundred years later the young field of evolutionary biology faced a major
crisis. In 1859, Charles Darwin published the first edition of The Origin of
Species, but that very same year Louis Pasteur began the experiments that