Panspermia--true or false?

Sir

Chandra Wickramasinghe and colleagues suggest that the causative agent of the severe acute respiratory syndrome (SARS) epidemic might have an extraterrestrial origin. Their unstated assumption is that the emergence of life and the sudden appearance of several global epidemics are the outcomes of a continuous bombardment of Earth with bacteria and viruses, originating in the interstellar grains and comets.

This theory is unlikely. Delivery of exogenous material to the Earth's surface is a well documented phenomenon. It includes extraterrestrial organic compounds present in carbonaceous chondritic meteorites and interplanetary dust particles, which seem to be related to cometary nuclei. However, there is no basis for the claim made by Wickramasinghe and colleagues that there is a daily influx of about 1 tonne of extraterrestrial microbes, which they assume corresponds to one hundredth of the daily infall of cometary material. The present dust infall is 4×107 kg per year and, although its organic composition is poorly understood, there is no evidence that it includes extraterrestrial prokaryotes or other forms of microbial life. The few individual molecules that have been characterised in interplanetary dust particles are polycyclic aromatic hydrocarbons, none of which are bona-fide biosignatures. Although this material could have played a part in the origin of life, its connection with extant evolutionary processes is tenuous at best, and probably has no relevance in the appearance of infectious diseases.

Wickramasinghe and co-workers also argue that the microorganisms they collected at high altitude lend support to their hypothesis of an extraterrestrial origin of epidemics. The organisms they found include two bacteria that are firmly placed within the bacillus and the staphylococcus clades on the basis of 16S rRNA sequence comparisons. The presence of these microbes at 41 km from the Earth's surface is consistent with the presence of different bacterial species and fungal spores collected from clouds, but cannot be evidence of an extraterrestrial origin. If life exists elsewhere in the Universe, it is extremely unlikely that it would have independently evolved macro-molecules, such as 16S rRNA, or other intracellular components homologous to those of their terrestrial counterparts. This would be especially true of viruses, whose dependence on the intracellular molecular machinery of their hosts to complete their biological cycles would make their survival and evolution within the terrestrial biosphere unlikely should they have an independent, extraterrestrial origin.

The comment that some epidemics “bear the hallmarks of a space incident component” is particularly amusing. Our inability to reconstruct the complete chain of events cannot be considered evidence of an extraterrestrial origin for a disease. In the case of SARS the development of the pandemic can be traced almost on a day-by-day basis.

Evolutionary analysis of several protein-coding features of the sequences of the SARS-associated virus has provided clear evidence of its phylogenetic affinities with other mammalian and avian coronaviruses. Reconstruction of viral evolution can be notoriously complicated, but the evidence suggests that the SARS-associated coronavirus jumped into the human population from felines, which are considered a delicacy by many Asiatic gourmets.