Major Events in Evolution♦ 4.6 bya – formation of the Earth (Precambrian)

♦ 3.5 bya – prokaryotic cells♦ 2.2 bya – eukaryotic cells♦ 600 mya – soft-bodied invertebrates♦ 500 mya – colonization of land plants (Paleozoic)

♦ 420 mya – jawless fish♦ 375 mya – bony fish, amphibians, insects♦ 325 mya – first seed plants, reptiles♦ 220 mya – cone-bearing plants (Mesozoic)

♦ 175 mya – dinosaurs abundant♦ 80 mya – angiosperms♦ 60 mya – mammals, birds, pollinating insects (Cenozoic)

♦ 30 mya – primate groups♦ 2.5 mya – apelike ancestors♦ 0.5 mya – humans appear

Major Events in Evolution

Life on Earth Between 4.0 and 3.5 Billion Years Ago • The planet was bombarded with by

huge rock bodies left over from the formation of the solar system.

• One of them may have dislodged a chunk of Earth that became the moon.

• The pounding may have generated enough heat to vaporize all the available water and prevent seas from forming.

• As Earth began to cool and liquid water could exist, prokaryotic life appeared.

Life on Earth Between 3.5 and 2.0 Billion Years Ago • The fossil record supports that

prokaryotes were the first organisms on the planet.

• Two main evolutionary branches to the prokaryotes existed that still exist today, bacteria and archaea.

Fossilized Ancient Bacterium Living Leptolyngbya Bacterium

Oxygen Began Accumulating in the Atmosphere About 2.7 Billion Years Ago

• The only photosynthetic prokaryotes that generate oxygen are called cyanobacteria.

• The free oxygen probably dissolved in the surrounding water until the seas and lakes became saturated with the oxygen.

• Once the dissolved iron precipitated from the water as iron oxide, O2 began to “gas out” of the water and accumulate in the atmosphere.

Eukaryotic Life Began By 2.1 Billion Years Ago

• The oxygenation of the atmosphere coincides with the evolution of eukaryotic cells.

• With the evolution of eukaryotic cells came chloroplasts and mitochondrion.

Multicellular Eukaryotes Evolved By 1.2 Billion Years Ago

• As eukaryotic single-celled organisms flourished, they gave rise to a diversity of protists.

• Multicellular eukaryotes also evolved and gave rise to modern day algae, plants, fungi, and animals.

Fossilized Algae Fossilized Animal Embryo

Animal Diversity Exploded During the Early Cambrian Period

• 600 mya – First invertebrate animals appear; porifera (sponges)

• 543 mya – First cnidarians; jellyfish and hydra

Plants, Fungi, and Animals Colonized the Land About 500

Million Years Ago

• Challenges that organisms face as they move from life in the water to life on land include preventing the loss of water and reproducing on land.

• Plants had to develop root systems to anchor themselves and take up water and nutrients.

• Plants also developed a cuticle to prevent water loss.

Plants, Fungi, and Animals Colonized the Land About 500 Million Years Ago

• The most widespread and diverse land animals are the arthropods (mostly insects and spiders)

• The land vertebrates, or tetrapods, evolved from marine fishes

– Amphibians evolved from fishes

– Reptiles evolved from amphibians

– Both birds and mammals evolved from reptiles

• Most groups of modern mammals were established about 50 - 60 mya including primates

• The human lineage diverged from other primates only 5 mya

The Origin of Life

• Biogenesis – life arises only by the reproduction of preexisting life; Pasteur’s experiment proved this.

The Origin of Life – How Can Life Arise From Nonliving Matter?

• Hypothesis for the origin of life1. The abiotic synthesis of small organic

molecules, such as amino acids and nucleotides2. The joining of these small molecules

(monomers) into polymers, including proteins and nucleic acids

3. The origin of self-replicating molecules that eventually made inheritance possible

4. The packaging of all these molecules into “protobionts,” droplets with membranes that maintained an internal chemistry different from the surroundings.

Hypothesis For The Origin Of Life• Oparin and Haldane independently

hypothesized in the 1920s that the early atmosphere was probably a reducing atmosphere (electron adding) because there was less oxygen

• In order for organic molecules to be produced, considerable energy must be provided –lightning and intense UV radiation could provide this energy

Hypothesis For The Origin Of Life• In 1953, Stanley

Miller and Harold Urey tested the Oparin-Haldane hypothesis

• They were able to produce a variety of amino acids and other organic compounds found in living organisms today

Hypothesis For The Origin Of Life• RNA may have been the first genetic material

– Short polymers of ribonucleotides have been produced abiotically in lab experiments

– RNA molecules act as catalyst in modern cells and may have been the first enzymes (ribozymes)

• RNA could have provided the template on which DNA nucleotides were assembled

• Once DNA appeared, RNA molecules would have begun to take on their modern roles

The Five-Kingdom System

• Whittaker’s five-kingdom system (1969)

• Classified living things on the basis of cell type and nutrition

The Three-Domain System