WHY DID THE DINOSAURS DIE?


Possible cause(s) of the extinction of the dinosaurs have long been debated. The possibility that fungi may have played a role has now been suggested."

Several bacteria and viruses can cause systemic infections in otherwise healthy individuals. Tuberculosis, small pox, plague, and many other bacterial and viral infections have been defined and are known since antiquity. Fungal infections, on the other hand, have been recognized more recently. Cryptococcus neoformans, for example, was not known to be associated with human disease until 1894 because cryptococcosis is rare and develops only in existence of some predisposing factors.

Based on their source, fungi can be divided into two groups. The first group (e.g. Candida, Malassezia) is a part of the flora and can lead to a systemic disease only if there is an alteration in the ecology and/or the immune status of the host.

The second group (e.g. Histoplasma capsulatum, C. neoformans), on the other hand, lives in soils and does not require the existence of any animal host for growth or survival. Exposure to large inocula of these fungi can lead to development of systemic disease even in otherwise healthy individuals.

The low incidence of systemic fungal infection despite the ubiquitous nature of fungi suggests that mammals were selected in evolution for defense against fungi. In addition, interactions between these fungi and other environmental microorganisms, including amoeba, slime molds, and worms, may possibly lead to the emergence and maintenance of characteristics that are needed for virulence and survival in animal hosts. Animal passage, on the other hand, may lead to the emergence of genetic and phenotypic changes and increases the diversity and ability to survive environmental changes.

So, what are the defense mechanisms that protect against fungal infections? The most important ones are the immune system and physical characteristics that include a high basal temperature and slightly alkaline serum chemistry. High basal temperature does not permit the growth of fungi since most grow best at ambient temperatures of 25-35°C. Alkaline nature, on the other hand, is not favorable for most fungal species since they grow best at acidic pH of 6-6.8. Furthermore, fever response may also reduce the likelihood of survival of the fungal species in mammalian host.

Ability to survive at mammalian temperatures are needed but of course not adequate for pathogenicity of the fungus. Adhesion to mammalian cells is needed for initiation of the infection and several other virulence factors, such as capsule formation, melanization, formation of hypha and yeast-hypha transition are related to pathogenicity and biological escape mechanisms for survival.

So, why and how all these are possibly related to dinosaurs? The reason for the extinction of the dinosaurs still remains unknown. The common belief is that after a meteor hit the Earth 65 million years ago, the life in the oceans was destroyed and the dinosaurs died. Interestingly, this meteor slam did not much affect the mammals.

Dr. Arturo Casadevall, an infectious disease researcher from Albert Einstein College of Medicine, New York recently proposed an interesting theory for this mystery. According to his theory, following the meteor hit, massive fungal proliferation was observed, possibly due to the decreased sunlight and cooler global temperatures. This eventually lead to the increase in the amount of the airborne fungal spores and animals and plants were exposed to large fungal inocula. Consequently, some animal species were infected and died. The warm-blooded animals and birds could surprisingly survive, possibly because their body temperature was high for the fungi to grow and lead to infection.

Thus, although difficult to prove, proliferation of fungal spores might have contributed to the extinction of the dinosaurs and the flourishing of mammalian species. This theory proposed by Dr. Casadevall now brings a new insight to the mysterious disappearance of the dinosaurs.

Related reading

1. Casadevall A. Fungal virulence, vertebrate endothermy, and dinosaur extinction: is there a connection? Fungal Genetics and Biology 2005; 42: 98-106.