Earth Science: Biology – Evolution

Discovery Science: Earth – Biology – Evolution

About 3.5 billion years ago, the first signs of life on Earth were single-celled bacterial organisms that lived in the primeval oceans. Single-celled animals and algae, as well as all multicellular organisms, evolved from these bacteria. This evolutionary process was limited to the oceans for many millions of years. Some species developed the ability to photosynthesize, a process by which oxygen is released as a waste product.

Over time, photosynthetically active organisms enriched the atmosphere with oxygen and gradually an ozone layer formed, protecting the Earth from the harmful ultraviolet radiation emitted by the sun. The conditions were now favorable for life to evolve on land. The first organisms to make the transition to terrestrial habitats were moss-like plants. Small invertebrates of the phylum Arthropoda followed.

Arthropods were the ancestors of spiders and insects. The first vertebrates were jawless fish with primitive fins. Amphibians evolved from air-breathing fish with lobe-shaped fins. These animals slowly occupied terrestrial habitats through the extensive swamps in dense forests. However, they remained in close contact with the aquatic environment, as they were forced to return to the protection of water to lay their eggs.

Reptiles were finally able to live independently from bodies of water with the evolution of hard-shelled eggs that prevented the dehydration of enclosed liquids. Plants too became less reliant on aquatic environments as they evolved specific vessels that transported water and nutrients throughout the tissues of the whole plant. Eventually, they also developed seeds, which allowed for a reproduction process fully independent of bodies of water.

During the following phase, reptiles expanded into every available habitat and took advantage of every ecological niche. While dinosaurs were spreading all over the Earth, the first mammals also appeared, but for many millions of years they were a rather small and insignificant group of animals. This changed about 65 million years ago, probably due to a large meteorite hitting the Earth. The meteorite filled the atmosphere with suspended dust, which blocked the sunlight and cooled down the Earth’s temperatures dramatically.

The dinosaurs, as well as many other animals, became extinct. However, as devastating as this catastrophe was, some terrestrial life continued and survived the subsequent major mass extinction events. The elimination of dominant species during such mass extinction allowed other animals, like mammals and birds, to finally occupy newly vacant ecological niches, and for life to continue evolving.

As warm-blooded animals that could maintain a constant, warm body temperature, mammals and birds were able to adapt to a very wide range of environmental conditions. The era of reptiles was replaced by the era of mammals whose dominant species became Homo sapiens. Humans evolved about 2.5 million years ago from their hominid ancestors, the australopithecines. Walking upright, this group of early humans left their original homeland in East Africa, and began their conquest of all the continents.