Positive disasters

The asteroid heads for Earth. Image source: Image by MasterTux from Pixabay.

By Jozef Klembara

Over the course of the last 550 million years or so, the evolution of the Earth has, with all its organismal worlds, been at certain intervals interrupted by drastic events called mass extinctions. The event that occurred between the Palaeozoic and Mesozoic eras of the Earth’s history (about 245 million years ago) is considered to be the most drastic one. As its consequence, about 95% of all marine species became extinct and the decrease of the diversity of all terrestrial vertebrates is estimated to have been about 50%! This was not an enigmatic “catastrophic” event, but probably a consequence of big climatic and paleo-geographic changes that occurred at the Palaeozoic-Mesozoic boundary, which accompanied the formation of the single southern continent of Pangea. After this event, Pangea started to split again into the individual continents, the shapes of which more-or-less correspond to those of today.

What followed after such drastic global destruction of almost the entire ecosystem in that period of the Earth’s history? We have some information, but it has been unclear 1) how biotic recovery proceeded, and 2) how quickly the recovery of the ecosystem took place. It means that the interrelationships between the individual elements (flora – fauna) of the ecosystem were not known. However, a recently published paper by Lyson and his colleagues (1) describes the continuous recovery of the ecosystem after the asteroid impact 66 million years ago that killed off not only the dinosaurs, but 75% of all living species, including mammals larger than a rat. About 50% of plant species died out too. The investigated locality in Texas preserves the complete sequence of sediments accumulated for one million years. 

The first surprising discovery is that plants and animals started to evolve much faster than previously thought. Second, the rapid evolution of plants was a spurring factor for such evolution and the great diversity of the mammals that replaced dinosaurs. The presence of thousands of well-preserved pollen and spores showed a surge in the evolution of ferns, which thrived in the devastated ecosystem. 

Before this catastrophe, the fossil record at that locality shows the presence of raccoon-sized mammals, but 1000 years after the impact, only very small mammals of about the size of a shrew roamed the world. The ferns prevailed; flowering plants, with their nutritive fruits and seeds, were rare. 100,000 years later, the mammals were back at the size of a raccoon and their species diversity was much higher. These mammals foraged in palm forests that replaced the previously abundant fern species. Over the next 200,000 years, during the so-called “palm period”, walnut-like plants developed. The consumption of their nutritious seeds significantly stimulated the evolution of mammals. Many new species evolved and their size was already that of a beaver (up to about 25 kg). At about 700,000 years, during the “protein bar period”, first legumes arose that provided protein-rich meals, which in turn boosted mammalian diversity and increased their size. Several mammalian species weighted about 50 kg and increased their size about 100-times relative to those that survived the asteroid impact. 

This is only a very short section of the Earth’s evolution, but it opens to us a door through which we can see much better how evolution “works”, how it “produces” still more advanced forms than those existing before an event of complete and utter devastation. 

References

  1. Lyson et al. 2019. Exceptional continental record of biotic recovery after the Cretaceous– Paleogene mass extinction. Science 366, 6468: 977-983.

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