Unit 3:  The origin and evolution of life                          1   2   3   4   5   6 

5. Extintion

At least the 98 % of all species that have existed along the history of the Earth are today extinct. Extinction and appearance of species are natural phenomena. The average time life of a species is around one million of years.

Several causes can produce the extinction:

-   Accumulation of neutral traits that transform one species into another same time later.

-   Disadaptation to the environment (due to climatic changes that modify the environmental conditions, disappearance of prey, etc.).

-   Excessive specialisation (makes the species less adaptable to changes, even the smaller ones) and short distribution (species are more vulnerable).

This type of extinction, known as background extinction, is constant and gradual. However there is another type of extinction called mass extinction, which affects a huge number of species in very short periods of time.  These global events are related to big natural catastrophes (meteor impacts, intensive volcanic activity periods, climatic changes, etc.). There proofs of five mass extinctions along the history of the Earth:

-   First extinction. It was 435 million years ago (Palaeozoic). A long glaciation almost ends with the marine life. Only some fish survive and invertebrates pay a big tribute.

-   Second extinction. It was 367 million years ago (Devonian). A big number of fish species and the 70% of marine invertebrates were wiped out.

-   Third extinction. It was 245 million years ago (Permian-Triassic). It is the most dramatic of all these major events. The 90% of all existent species disappeared (e.g. Trilobites).

-   Forth extinction. It was 210 million years ago (Triassic). The 75% of the marine invertebrates disappeared, and the mammal-like reptiles extinct, giving step to dinosaurs.

-   Fifth extinction. It was 65 million years ago (Cretaceous). Among others, dinosaurs, and ammonites disappeared. The mammals and birds Era starts.

Nowadays, many scientists advert about the imminence of the sixth mass extinction.

In contrast with the previous ones, it is a phenomenon caused by human beings because of the systematic and constant destruction of species in our modern world due to activities such as alteration and degradation of ecosystems, the overexploitation of natural resources, the human overpopulation, the climatic change, etc.

If we keep in this way, the sixth extinction will be soon a fact.

5.1. Which characteristics makes a species especially vulnerable

       or prone to extinction?