Ch 12: The History of Life

The geologic time scale divides Earth’s history based on major past events.

Geologic Time Scale• Is a representation of the history

of the Earth• Organizes Earth’s history by major

changes or events that have occurred, using evidence from the fossil and geologic records.

Organization of the Geologic Time Scale

Divided into a series of units based on the order in

which different groups of rocks and fossils were

formed.

3 basic units:

1. Eras- – last tens to hundreds of millions of years– consist of two or more periods– three eras: Cenozoic, Mesozoic, Paleozoic

2. Periods-– most commonly used units of time on the scale– lasting tens of millions of years– Each period is associated with a particular type of rock

system.

3. Epochs-– smallest units of geologic time– last several million years

Multicellular life evolved in

distinct phases.

Paleozoic Era• Multicellular organisms first appeared during

the Paleozoic era.• The era began 544 million years ago and

ended 248 million years ago.• The Cambrian explosion led to a huge diversity

of animal species.

Mesozoic era• known as the Age of Reptiles.• It began 248 million years ago and ended 65

million years ago.• Dinosaurs, birds, flowering plants, and first

mammals appeared during this time.

Cenozoic era• First appearance of mammals• The Cenozoic era began 65 million years ago

and continues today.• Placental mammals and monotremes (lay

eggs) evolved and diversified.• Anatomically modern humans appeared late

in the era.– This is YOU!!

Primates• Primates are mammals with flexible hands and

feet, forward-looking eyes and enlarged brains.• Also have arms that can rotate in a circle

around their shoulder joint, as well as thumbs that can move against their fingers.– Opposable thumbs

• Include:– Lemurs, monkeys, apes and humans

• Primates share physical traits and molecular similarities.

Evolutionary Relationships of Primates

• Primates are divided into 2 groups:– Anthropoids– Prosimians

• Anthropoids are divided into:– Hominoids– Monkeys

• Hominoids are divided into:– Lesser apes (gibbons)– Great apes (orangutans, chimpanzees, and gorillas)– Hominids (include humans)

Hominid Species• Classified into 2 groups:– genus Australopithecus– genus Homo

• Australopithecus afarensis– Lived 3-4 million yrs ago in Africa– Smaller brain, humanlike limbs

• Homo habilis– Lived 2.4-1.5 million yrs ago in modern day Kenya and

Tanzania– Earliest known hominid to make stone tools– Brain was larger and shape was more similar to that of

a modern human

Hominid Species• Homo neanderthalensis– Lived 200,000 to 300,000 yrs ago in Europe and

Middle East• Homo sapiens– Includes modern humans– First appeared in Ethiopia around 100,000 yrs ago

( according to fossils found there)– Different features from modern humans• Proof that the species did not stop evolving

Examples of Hominid Skulls

• Australopithecus• afarensis

• Homo habilis • Homo • neanderthalensis

• Homo sapiens