Origin of Life

Chapter 12

KEY CONCEPT The origin of life on Earth remains a puzzle.

Earth was very different billions of years ago.

• There have been many hypotheses of Earth’s origins.

• The most widely accepted hypothesis of Earth’s origins is the nebula hypothesis.

Several sets of hypotheses propose how life began on Earth.

• There are two organic molecule hypotheses.– Miller-Urey experiment

– meteorite hypothesis

electrodes

heat source amino acids

water

“atmosphere”

“ocean”

• There are different hypotheses of early cell structure.– iron-sulfide bubbles hypothesis

• There are different hypotheses of early cell structure.– lipid membrane hypothesis

• A hypothesis proposes that RNA was the first genetic material.

– Ribozymes are RNA molecules that catalyze their own replication.

– DNA needs enzymes to replicate itself.

Microbes have changed the physical and chemical composition of Earth.

• The oldest known fossils are a group of marine cyanobacteria. – prokaryotic cells– added oxygen to

atmosphere– deposited minerals

• Fossil stromatolites provide evidence of early colonies of life.

Eukaryotic cells may have evolved through endosymbiosis.

• Endosymbiosis is a relationship in which one organism lives within the body of another.

• Mitochondria and chloroplasts may have developed through endosymbiosis.

The evolution of sexual reproduction led to increased diversity.

• Genetic variation is an advantage of sexual reproduction.• Sexual reproduction may have led to the evolution of

multicellular life.

Life moved onto land during the 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.

• Life moved onto land in the middle of the Paleozoic era.

Reptiles radiated during the Mesozoic era.

• The Mesozoic era is 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.

Mammals radiated during the Cenozoic era.

• The Cenozoic era began 65 million years ago and continues today.

• Placental mammals and monotremes evolved and diversified.

• Anatomically modern humans appeared late in the era.

Humans share a common ancestor with other primates.

• Primates are mammals with flexible hands and feet, forward-looking eyes and enlarged brains.

• Primates evolved into prosimians and anthropoids.– Prosimians are the oldest living primates.– They are mostly small and nocturnal.

– They are subdivided into the New World monkeys, Old World monkeys, and hominoids.

– Anthropoids are humanlike primates.

– Homonoids are divided into hominids, great apes, and lesser apes.

– Hominids include living and extinct humans.

• Bipedal means walking on two legs.– foraging– carrying infants and food– using tools

• Walking upright hasimportant adaptiveadvantages.

There are many fossils of extinct hominids.

• Most hominids are either the genus Australopithecus or Homo.• Australopithecines were a successful genus.• The Homo genus first evolved 2.4 million years ago.

Modern humans arose about 200,000 years ago.

• Homo sapiens fossils date to 200,000 years ago.• Human evolution is influenced by a tool-based culture.• There is a trend toward increased brain size in hominids.

Australopithecusafarensis

Homo habilis Homo neanderthalensis

Homo sapiens