19 Feb 2003
(Source: NASA Astrobiology Institute)
Featured Question: A few thoughts on Mass Extinctions
By Madalyn Edwards and Daniella Scalice
NASA Astrobiology Institute
February 19, 2003
Q: Kindly give me an overview of extinction:
(1) Why do extinctions happen?
Extinction is a frightening concept at first glance. Why do life forms that are vibrant and thriving one minute end up on the cutting room floor the next? How can magnificent creatures such as dinosaurs be cast as extras while the common cyanobacteria continue to receive top billing? In truth why extinction occurs varies greatly. A species may become extinct because the environment that supports its ecological niche is no longer able to sustain a rapidly growing population. Or a new predator may stumble upon a juicy find and wipe out an entire species in one fell swoop.
On a larger scale, numerous species have found themselves on the wrong end of a bad weather front. Climatic changes have initiated many extinction events since life first evolved on Earth. Ice ages (glaciation events), volcanic eruptions, and changes in sea level appear to be the most common culprits. Since species often selectively adapt to particular environments, even subtle changes can leave many struggling to survive. Overall this is a natural process that aligns nicely with the theory of evolution; as one species faces its last scene another gets ready for its close up.
(2) Examples of mass extinctions? (3) What became extinct?
Officially there have been five big mass extinction events over the past 540 million years. What is often not mentioned is that up to five other mass extinctions occurred between 650-500 million years ago. These little publicized events mainly involved microorganisms, and marine animals and plants. They each took place before the Cambrian Explosion (between 543 and 490 million years ago) during which time life forms on Earth exploded into previously unseen levels of diversity. These smaller Pre-Cambrian extinctions were potentially less detrimental than the five main Post-Cambrian events because there were relatively fewer species on the planet to become extinct.
The Earth has not seen the same type of diversification of life forms since the Cambrian Explosion, so the percentage of species lost with these five main extinction events is quite stunning. Between 75-95% of all species were lost with each extinction event. The ecological niche most commonly affected by these mass extinctions involved sea dwelling creatures. Slight changes in temperature, oxygen level, or the sea level itself can and has greatly affected marine life.
(4) Advantages and disadvantages of extinction?
It is probably quite doubtful that expiring species find any comfort in the fact there may actually be an advantage to their demise. The general advantage to an extinction event is that other species are allowed to proliferate due to the loss of a food source competitor or even a predator. Case in point: we humans did not start our evolutionary pathway until many of the large mammals that had dominated the lands became extinct. The disadvantage to extinction is of course that once a species makes its exit, there can be no encore performance. In today's world, species that have yet to be discovered are being lost and their roles in the ecosystem can not be replaced by just any bit player. The part they played was written for them and them only. The cost of losing these characters may take years to understand.
(5) Big questions still to be answered on extinction?
The phenomenon of mass extinctions sets the stage for many questions that have yet to be answered with complete certainty. One big question is whether there is a cyclical pattern to mass extinction events. Within the fossil record - an incomplete script at best - there appears to be a pattern that suggests mass extinction events occur every 26-30 million years. This pattern is thought to be related to celestial objects such as comets and meteors which have long been known to travel distinct paths on very dependable timetables. This could imply that there have been possibly up to 23 mass extinction events since life first evolved on Earth!
(6) The future about extinction - what might happen - when might it happen - why might it happen?
Presently, the certain mystery of future mass extinction events has yet to be solved. Is extinction a predictable phenomenon, or are we at the mercy of a random catastrophe which will burst onto the scene unannounced and carry us away with our neighboring species? How capable are we humans of truly creating a mass extinction, let alone preventing one?
We are indeed all subject to the uncertainty of cause and effect - will our candle be snuffed out before we discover how to guard our flame indefinitely, or will an ill fated breeze extinguish the light of life on Earth forever?
This beautiful blue stage we call home often offers more questions than answers.