The Geologic Time Scale is just a classification that geologists have developed to classify time into geologically meaningful chunks. The largest units of time are called Eras, which are broken down into smaller units called Periods, and then into even smaller units called Epochs. (A version of the Geologic Time Scale is found below, and also in your text on p.5.) Archaeologists use the geologic time scale, but they also use the archeological time scale (Paleolithic etc.) discussed in a later lesson, and also in the text..
The Geologic Time Scale was originally developed by observation of the stratigraphy in different areas in the world, and the realization that many of these layers of deposits, or strata, were put there by the same forces (some gradual, some catastrophic) that we can see in action today. This is the principle of uniformitarianism. As noted in the previous lesson, the law of superposition states that in most cases, the older strata will be on the bottom, and the younger strata will be on top.
By observing the strata, and the fossils contained within them, geologists used cross-dating to develop the sequence of the Geologic Time Scale as a way to determine the relative age of different strata. Although it was clear by the 1830's that long periods of time would be necessary for the sequence of deposits (as well as erosion) to have occurred, it wasn't until the development of absolute dating techniques in the last half of the twentieth century that anyone really knew how old was old.
The results have been somewhat astounding. The earth itself is in the vicinity of four and a half billion years old, but life in any form did not appear for another billion years. Our modern era, the Cenozoic, did not start until about 65 mya, along with the appearance of the first prosimian-like Primates. The Cenozoic is also the time period when most mammals evolved, becoming one of the more numerous of the animal classes, and certainly the class that contained the largest animals. However, if humans were the point of evolution on earth, it took an awfully long time to get to it: bipedal Primates of any sort didn't appear until slightly over 4 million years ago, and our species Homo sapiens did not appear until 200,000 years ago. Be sure to spend some time studying the Geologic Time Scale.
| ERA | PERIOD | EPOCH | YEARS AGO (IN MILLIONS) | MAJOR EVOLUTIONARY EVENTS |
|---|---|---|---|---|
| Cenozoic (Age of Mammals) | Quaternary | Holocene | .1 | Farming begin |
| Pleistocene | 1.8 | Various hominin species at beginning; Homo sapiens evolves and spreads; other hominins disappear | ||
| Tertiary | Pliocene | 5 | Old World Monkeys adaptive radiation; Apes decrease; 1st bipeds and small hominin adaptive radiation; 1st stone tools at 2.5 mya | |
| Miocene | 23 | 1st Old World Monkeys; Adaptive Radiation of Hominoids (Age of the Apes) | ||
| Oligocene | 35 | Radiation of Anthropoids; 1st New World Monkeys (27 mya) |
||
| Eocene | 56 | Adaptive Radiation of Prosimians, but most are extinct by end of Eocene; Shoshonus, early tarsier, at 50 mya; 1st anthropoids, e.g. Eosimias (45 mya) and Baliina (40 mya) | ||
| Paleocene | 65 | Adaptive Radiation of placental mammals; 1st Primates: prosimians | ||
| Mesozoic (Age of Dinosaurs) | Cretaceous | 146 | Flowering plants; mammals and birds spread (76% of all species become extinct at end of Mesozoic) | |
| Jurassic | 208 | Many dinosaurs; primitive mammals spread; toothed birds | ||
| Triassic | 245 | 1st dinosaurs; 1st egg laying mammals | ||
| Paleozoic | Permian | 290 | Spread of reptiles, insects, 1st mammal like reptiles (96% of all species become extinct at end of Paleozoic) |
|
| Carboniferous | 363 | Reptiles, modern insects (82% of species become extinct 380,000,000 years ago) | ||
| Devonian | 409 | 1st amphibians; forests appear | ||
| Silurian | 439 | 1st land plants | ||
| Ordovician | 510 | Invertebrates; 1st vertebrates (85% of species become extinct 440,000,000 years ago)* | ||
| Cambrian | 570 | Invertebrates | ||
Proterozoic (earliest life forms) |
3,000 | Protozoa, sponges, algae |
||
| 3,500 | 1st life (bacteria) | |||
| Azoic (No Life) | 4,550 |
Origin of Earth |
* At least five catastrophic events(and at least eight lesser ones) have occurred which caused wide-spread extinction. The last but one occurred at the end of the Mesozoic, when 75-76% of all species died out, including all the large reptiles, probably due to climatic change caused by an asteroid hitting earth. The reptiles vacated many econiches which soon became occupied during the adaptive radiation of placental mammals. We are currently living in the most recent catastrophic event. Modern humans are causing widespread extinction, perhaps at the rate of as many as 100 species per day.
For more information of the Geologic Time Scale, check out the USGS site on the subject.
Since I don't want you to miss it, and it is certainly relevant to the evolutionary events presented on the right in the Geologic Time Scale, check out where the continents were at different times: http://pubs.usgs.gov/gip/dynamic/historical.html