A Brief History of Life on Earth An Overview of Events That Have Shaped Life on Our Planet

To better understand the animals that inhabit our present-day world, it helps to examine the broad history of life. By grasping how organisms, past and present, have evolved and diversified throughout the history of our planet, we can better appreciate the animals and wildlife that surround us today.
In this article, we'll examine the major events that took our planet from its lifeless molten beginnings to the present day and look at how scientists break down our planet's 4.5 billion year history into eons, eras, and periods.
It all began 4500 million years ago (Mya) with the Hadean Eon. That's when our solar system formed and, in that solar system, a molten Earth started to cool and take shape. Because there was no solid rock back then, we don't have a geological history of the Hadeon Eon. As soon as the Earth cooled and solid rock formed, that's when geological history began and the Hadean Eon ended.
The Hadeon Eon was followed by the Archean and Proterozoic Eons (often refered to collectively as the Precambrian). It was during the Archean Eon (3800-2700 Mya) that the first organic lifeforms developed out of a mixture of inorganic compounds. The oldest known prokaryotic fossils date back to the Archean Eon. The Proterozoic Eon (2700-590 Mya) gave rise to the first eukaryotic fossils (algae).
Things really started to get interesting at the beginning of the Phanerozoic Eon (590 Mya-present). The Phanerozoic Eon is the current eon and so much has happened thus far during this eon that scientist have broken it down into smaller pieces to describe the events. So, let's first look at how the Phanerozoic Eon has been broken down. The Phanerozoic Eon is broken down into the following three eras:

• Paleozoic Era (590-248 Mya)
• Mesozoic Era (248-65 Mya)
• Cenozoic Era (65 Mya-present)

The Paleozoic Era (590-248 Mya) is an exciting time in the history of life that begins with aquatic life and ends with a diverse collection of organisms including insects, amphibians, plants, fishes, and reptiles. Scientists divide the Paleozoic Era era into six periods:

• Cambrian (590-505 Mya) - aquatic life only, all modern phyla of animals develop, first vertebrates arise, triolobites abundant
• Ordovician (505-438 Mya) - still only aquatic lifeforms, fishes diversify, invertebrates diverse and domanate; first mass extinction event
• Silurian (438-408 Mya) - jawless fishes diversify, jawed fishes develop, first land plants and animals arise
• Devonian (408-360 Mya) - first amphibians, land organisms diversify, first bony fishes, first sharks; second mass extinction event
• Carboniferous (360-286 Mya) - first reptiles, first conifers, seed ferns abundant, coral deposits
• Permian (286-248 Mya) - insects, reptiles diversify, first therapsids, cycads and conifers diversify; third mass extinction event

The Mesozoic Era (248-65 Mya) saw the first mammals but without a doubt, was the era that belonged to the reptiles. Scientists divide the Mesozoic Era into three periods:

• Triassic (248-213 Mya) - reptiles diversify, first dinosaurs appear, bony fish diversify; fourth mass extinction event
• Jurassic (213-144 Mya) - dinosaurs dominate, first mammals, first birds, gymnosperm forests
• Creataceous (144-65 Mya) - modern birds, modern fishes, angiosperms, climax of dinosaur domination, period ends with the extinction of the dinosaurs; fifth mass extinction event

The Cenozoic Era (64 Mya-present) is the age of mammals. Scientist divide the Cenozoic Era into two periods:

• Tertiary (65-1.8 Mya) - placental mammals, modern mammals, primates, grazing mammals, origin of the human family
• Quaternary (1.8 Mya-present) - modern humans worldwide distribution and dominance; sixth mass extinction event