Which span of geologic time is the longest

Remember relative time units often mark the first appearance of a biologic organism. This would be similar to what event in your life? Take another look at the geologic time scale. Notice that an absolute numerical age has been assigned to the beginning of each eon, era, period, epoch and age.

These have been determined by a variety of methods, the most common of which is the radiometric dating of igneous rocks. Think of radiometric dating methods as providing a birth certificate for the rock, much like the one issued to your parents when you were born.

Geologic ages are recorded in years before present, so that the Precambrian began 4, million years ago. Assign an age to the beginning of each era and period of your family time scale. See My Relatives Time Scale for an example. Download Activity Materials. Explore This Park. Student Activities Geology, Relatives, and Time.

Fossil Butte National Monument. Overall Rating Add your review. Tags: geology geologic time. The first animals to begin to colonize dry land were likely arthropods Fig.

Arthropods are a group of invertebrate animals with jointed arms and hard shells including crabs, shrimp, spiders, and insects. By the end of the Paleozoic era—almost mya after the Cambrian explosion—arthropods, amphibians, and some primitive reptiles dominated the terrestrial environment Fig. The cause of this extinction event remains unresolved, however scientists have proposed several possible scenarios including asteroid impacts and catastrophic volcanic events.

This extinction event is one of many throughout the history of life on Earth. The land was covered with non-flowering gymnosperm plants, the ancestors of modern-day conifer trees commonly known as evergreen or pine trees. The tree species Ginkgo biloba evolved during the Mesozoic era and survives to this day Fig.

It was also during this time that scaly-skinned reptiles replaced amphibians as the dominant vertebrate animals on land. Reptiles are a large and diverse group of vertebrate animals. Most of the reptile groups that lived in the Mesozoic era are extinct today. The best-known extinct reptile group is the dinosaurs. Dinosaurs represent a diverse group of reptiles with many different sizes and shapes Fig.

Dinosaurs and other reptile groups such as flying pterosaurs Fig. Despite depictions in popular Hollywood films, Tyrannosaurus rex would never have dined on a Stegosaurus or Brachiosaurus Fig.

Those dinosaur species were extinct long before the evolution of T. The Mesozoic era and the long reign of the dinosaurs ended around 66 mya with another dramatic mass extinction in which most of the dinosaurs were wiped out. The extinction of the dominant dinosaurs cleared the way for a previously obscure group to flourish: the mammals. This geological era spans from 66 mya to the present day. The human species is an example of a Cenozoic era mammal species.

Although the ancestors of early mammals first evolved in the late Paleozoic era and coexisted with dinosaurs during the Mesozoic era, this group did not flourish until the Cenozoic era that began 66 mya.

During the Cenozoic era, mammals became much more diverse, from small shrews, rodents, and bats to large bears, whales, and apes Fig. During the late Cenozoic era, around two million years ago, the first early human species evolved from an ape ancestral species. Anatomically modern humans Homo sapiens first appeared in the fossil record around , years ago.

All of known human history falls within the previous 10, years—a very short time compared to the rest of the geological timescale Table 7.

The earth has changed a great deal over its long 4. It has gone from a molten ball of elements to a layered rocky planet. The history of life on Earth, from microscopic bacteria to enormous whales, reflects the many changes the planet has undergone. The fossil record provides strong and abundant evidence of several mass extinctions of throughout the history of the planet.

These extinction events indicate sudden, catastrophic changes on Earth. For example, the boundary between Paleozoic and Mesozoic eras between Permian and Triassic periods is marked by the extinction of over 90 percent of all species living at the time.

Similarly, the end of the age of reptiles and dinosaurs was marked by the Cretaceous-Paleogene extinction event around 66 mya. Scientists have long sought evidence of the causes of these mass extinctions. It is hypothesized that an asteroid impacting Earth caused the decline of the dinosaurs during the Cretaceous-Paleogene extinction.

This hypothesis is supported by evidence from ancient craters, rocks enriched with rare minerals found only in meteorites, and rock formations indicative of major impact. As species evolve or go extinct, the fossils trapped in the rock layers reflect these shifts. How to track such a long, complex history? Using dazzling detective skills, geologists created a calendar of geologic time. They call it the Geologic Time Scale. The oldest — and by far the longest — is called the Precambrian.

The Cenozoic started about 65 million years ago. Each of these Eras, in turn, are divided into increasingly smaller divisions known as Periods, Epochs and Ages. That means changes happen in spurts, rather than at some slow and steady pace. Take the Precambrian Era. That burst marked the beginning of the Paleozoic Era. Sea creatures like trilobites and fish emerged and came to dominate. Then, million years ago, the Mesozoic Era burst into being. It marked the biggest mass extinction of all. It also kicked off the spread of life on land.

This era then ended abruptly — and famously — But they did understand relative ages, based on a simple, yet powerful principle. That principle is called the Law of Superposition. It states that in an undisturbed stack of rock layers, the oldest layers will always be on the bottom, and the youngest on top. The Law of Superposition allows geologists to compare the age of one rock or fossil to another.

It makes the sequence of geologic events more clear. After all, they lived millions of years apart. Still, how can we make sense of a calendar with no dates on it? To assign such absolute ages to the Geologic Time Scale, scientists had to wait until the s.