UV radiation: cause of mass extinction

THE MYSTERY about the cause of the last mass extinction at the Cretaceous-Tertiary, ie, the K-T boundary 65 million years ago, remains till today.

The main theories, among many, are the asteroid impact and volcanism. None of the theories so far, has been able to explain two unassailable facts, viz,

1. The dinosaurs alone were selectively wiped out; and 2. a host of other beings not only survived, but also evolved during the same period. Obviously, the survivors had protection against whatever agent it was that the dinosaurs were vulnerable to.

Therefore, the clue to the killing agent can be deduced from the protective mechanisms of the survivors. Surprisingly, all the creatures living today that came through the K-T boundary unscathed can be found to have protection against ultra- violet radiation. One of the important clues to that killer of eons ago comes from our eye.

Nictitating membrane

All mammals and other creatures like birds, reptiles and amphibians have nictitating membranes. The nictitating membrane in humans is reduced to a small vestigial, semi lunar fold, and the `plica semilunaris', at the inner corner of our eyes.

`Pterygium' (a very common eye condition in India), is a membranous growth across the cornea from the region of the plica semilunaris. The incidence of pterygium is confined to those latitudes of the planet between 23 N & S, which receive the most ultra-violet radiation. The growth of pterygium is an atavistic attempt to protect the cornea from UV radiation by our vestigial nictitating membrane.

Monkeys and the apes would have nictitating membranes and the lower primates have very rudimentary ones. Nictitating membranes protect against UV radiation.

All the other creatures that survived the K-T boundary can be seen to have a tough transparent protective covering for the cornea.

UV rays, insects and flowers

The Cretaceous saw the great development of the angiosperms the flowering plants, and along with them, the insect pollinators. An odd feature is that they all have visual pigments which enable them to see in UV light. Many flowers have alluring patterns visible only in UV light. When the visible spectrum is quite enough today for photopic (day-time) and colour vision, why have the faculty of UV perception? This is strongly indicative that there was an abundance of UV light when these insects and flowers evolved.

We associate feathers with flight. Feathers would have originally evolved as a skin appendage against the Sun's radiation and later on modified and adapted for flight.

The protective covering of feathers along with the nictitating membrane would have enabled the avian ancestors, i.e., the feathered dinosaurs to survive the UV radiation at the K-T boundary.

Hair, melanin and erect posture in humans

Humans, unlike other mammals, evolved long after UV levels had returned to present levels. Further, man's erect posture meant lesser body exposure to the Sun's radiation. Being the most exposed part, we have retained hair on the head. Hence, the protection against UV radiation was taken over by melanin pigmentation.

The amount of body hair, as is the melanin pigmentation, differs in different races. Higher the UV radiation, higher is the pigmentation, and vice-versa. Hence, hair is not a response to temperature, but rather to the latitude and therefore, the relative levels of UV radiation.

Peoples of the Middle East, the Mediterranean, the Central Asian and the North Indian regions though relatively fair skinned have abundant body hair. Lacking the protection of melanin, the hair may be trying to take over as a vestigial response to radiation. Therefore, one may infer that body hair in mammals was indeed evolved as a protection against radiation.

The increased UV radiation could have been due to atmospheric changes brought about by a number of causes like volcanism, or even some overactivity in the Sun itself. Two of the most extensive volcanic outpourings occurred about 67 million years and 54 to 57 million years back respectively.

One can only guess the huge amount of green house gases emitted had affected the ozone. It would have lasted through several million years, allowing for adaptation and evolution in some species.

For some reason, the dinosaurs were unable to adapt and so were selectively wiped out. The asteroid impact would just have been the final nail in the coffin.

D. Mathew Joseph

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