17-3 Evolution of Multicellular Life

The major spans of time on our planet can be divided into 4 major Eras. The first of these Eras is called Precambrian Time, and represents up to 90% of Earth’s history.

Several important events occurred during the periods of the Paleozoic Era. 1. The Ordovician and Silurian Periods- the first vertebrates. 2. The Devonian- first land animals 3. Carboniferous and Permian- first reptiles and leafy plants

A mass extinction at the end of the Paleozoic enabled dinosaurs to become the dominant species during the Mesozoic. Also, flowering plants appeared.

Following the mass extinction of the dinosaurs, the Cenozoic Era marked the appearance of the first mammals. Homo sapiens probably first evolved about 100,000 years ago in Africa. We are the last of our kind, extinct humanoid species include Homo neanderthalensis, Homo hominidae, Homo erectus, and others.

17-4 Patterns of Evolution

After an extinction event occurs, many habitats are left void of animals and many niches are left unfilled. This presents an opportunity for new species to colonize and evolve.

When a new species has the opportunity to evolve, they radiate from the point of origin, guided by Natural Selection. This is called Adaptive Radiation.

Adaptive radiation can produce organisms that are very similar in habits and appearance, but are completely unrelated species. This is called convergent evolution.

Coevolution can also result from adaptive radiation. This is when two closely associated species evolve in response to each other.

There are two hypotheses about general trends in evolution. Punctuated equilibrium is represented by long, stable segments interrupted by short periods of rapid change. The hypothesis of Gradualism states that things evolve over long, steady increments of time.

Genetic changes can also cause major transformations. The “master control genes” of the body are called hox genes and they guide major body developments.