Comets and the Origin and Evolution of Life

16 J. Oró, A. Lazcano, and P. Ehrenfreund
annihilating collisions is ridden with major problems, which include the lack
of detailed calculations of the energy associated with the lunar and terrestrial
cratering record, the difficulties involved in computations of the size of the
lunar impactors, and the absence of detailed models of the early Earth (Sleep
et al., 1989; Chyba, 1990; Zahnle and Sleep, 1997).
The geological record holds no evidence of catastrophic sterilizing event,
but it has been suggested that the memory of such intense heating is found
in the phylogenetic distribution of hyperthermophily among extant organisms.
Recent attempts to isolate thermophilic organisms have been extremely
successful, and have led to the characterization of prokaryotes that can live
and reproduce in abyssal environments with temperatures as high as 105–
110 ◦ C (Huber et al., 1989; Gottschal and Prins, 1991; Daniel, 1992; Johnson
et al., 2004). Research on extremophiles is important for providing insights on
the habitability of places beyond the Earth and the solar system. However, all
the available evidence suggests that the most basic questions pertaining to the
origin of life relate to much simpler replicating entities, predating by a long
(but not necessarily slow) series of evolutionary events the oldest recognizable
lineages (presently hyperthermophiles) represented in molecular phylogenies.
As the solar system matured, leftovers from the accretion period became
less abundant and the likelihood of giant collisions negligible. Massive collisions
may have delayed the origin of life, but their sterilizing effects were not
repeated in later times as shown by the continuity of the fossil record since
3.5×10 9 years ago. Once the Precambrian environment became oxidizing (Holland,
1994), a collision would probably lead to an extended destruction of the
reduced chemical species present in the cometary nuclei. However, the conversion
of the impactor’s kinetic energy into heat would cause major upheavals in
the terrestrial environment, and could be the explanation underlying several
major Phanerozoic extinctions (Steel, 1997).
The possibility that impacts of extraterrestrial bodies with the Earth may
influence biological evolution and led to the extinction of different taxa has
been discussed in the scientific literature for some time. As summarized by
Raup (1986), in 1970, Dewey M. McLaren suggested that the 365-million-yearold
mass extinction that marks the end of the Frasnian stage of the Devonian
period could have been caused by a giant meteorite. A few years later, a comparison
of tektite ages and geological periods led Urey (1973) to argue that
the different major extinctions that have taken place in the past 50 million
years could have been caused by cometary impacts, but this bold suggestion
went unnoticed. It was not until the discovery of iridium anomalies in different
parts of the world where Alvarez et al. (1980) hypothesized that a large
asteroid or comet collided with our planet and caused the extinctions that
marked the end of the dinosaurs 65 million years ago. According to this idea,
the end of the Cretaceous period was due to the impact of an extraterrestrial
object that threw large amounts of dust into the upper atmosphere for several
months, therefore initiating a chain reaction that began with a darkened
world and subsequently led to the cessation of photosynthesis, subfreezing