Endless ways
We do not have a conclusive definition of what is a species and perhaps this will always be a debate. Besides, this definition cannot be the same for microorganisms and animals. In most cases, there are several ways to distinguish species. All individuals of a same species are more similar to each other than to other species. This resemblace is not only morphological.
The individuals that belong to the same species are genetically closer, so they can only reproduce among themselves. Species are reproductively isolted from each other.
All current species come from others who lived in the past.
Species are basic units of biological classification. They also represent the unity of diversity in nature.
Currently, we estimate there have been described around 1,500,000 and 2,000,000 different species. Other estimations calculate 10,000,000 and 100,000,000. Each species represents an unrepeatable collection of genes and adaptations. Each one of them represent the end of an evolutive linage which, without interruptions, would bring us to the origins of life.
Each species plays a role in the ecosystem.
When a species is introduced or elliminated, the consecuences can be devastating.
Messages from the past
Fossils are the remainders or signs of the activity of organisms in the past which have come to the present in a preservation state that allows us to identify them. They represent the testimony of how life has changed over time and the appearance and extinction of species. One day, humans will be a fossil species too.
The fossil record is a very important tool for the reconstruction of evolutionary history and it includes remainders of species like microscopic organisms or even colossal mammals and reptiles. Although the fossil register of animals began only 600 million years ago, the vestige of life on Earth can we traced back to rocks that are more than 3 billion years old.
