Posted by Kasra
The Cretaceous mass extinction is one of the most exciting topics in evolutionary biology. There are always discussions on what caused the mass extinction, what happened during the extinction, what happened to all the dinosaurs, why did the mammals and birds survive, and so on. A recent article in PLoS Pathogens by Arturo Casadevall brings forward an interesting hypothesis: Fungi might have given mammals an evolutionary advantage during this period of time.
Casadevall mentions that humans and other mammals are generally resistant to fungal infections and most pathogenic fungi are in fact only opportunistic pathogens. He suggests that this is due to the mammalian control of body temperature – higher than optimal growth temperature of fungi – and evolution of powerful adaptive immunity. On the other hand, he brings forward examples of amphibians, being ectothermic, and primitive mammals, having lower body temperature, are more susceptible to fungal infections.
He next states that the Cretaceous-Tertiary (K-T) boundary included a cooling period with plenty of dust in the atmosphere and lack of sufficient sunlight. This led to a fungal bloom on the Earth and possible growth of pathogenic fungi. The hypothesis states that this higher than normal presence of fungi selected against surviving ectothermic reptiles and in favour of endothermic mammals. Thus, fungi might have indirectly helped mammals and possibly warm-blooded birds by killing off their competitors for the limited food resources.
Like any other scientific hypothesis, this one also needs to be tested. Unfortunately fossil records cannot tell us much of how common fungal infections were at the time. However, one can first look more closely if warm-bloodedness or higher body temperature does indeed aid in protection from fungal infection, given similar immune systems. Comparing today’s amphibians, reptiles and mammals can be tricky as the host-pathogen interactions may greatly differ among the groups. This could lead to conclusions that are confounded by differences in pathogenicity of fungi or power of the host immune system. Pooling data together from larger numbers host-pathogen pairs can lead to more robust conclusions. Application of heat-resistant fungi can also be beneficial for performing more controlled experiments rather than comparing natural histories. Casadevall himself suggests that climate change can promote evolution of fungi that can better survive in elevated temperatures and be threats as emerging pathogens. In this case, we would be a step in advance knowing what to expect, should these new pathogens emerge.
Casadevall A (2012). Fungi and the rise of mammals. PLoS pathogens, 8 (8) PMID: 22916007