Chapter 21 The Origin & Evolution of Life

I. Life Begins

A. Evolution of life has been linked, from its origin to the present, to the physical and chemical evolution of the earth.

B. Origin of Life

1. Early Earth and Its Atmosphere

a. Planets formed about 4.6 billion years ago.

b. By 4 billion years ago, the earth’s atmosphere (no oxygen) was forming, liquid water was being retained by gravity, and primitive living cells were emerging.

c. Is it possible to devise experiments that would test whether life could have originated spontaneously under primitive earth conditions?

2. Synthesis of Biological Molecules

a. Components for building biological molecules accumulated in the primitive earth; and energy (lightning, heat) was present.

b. Stanley Miller used a lab apparatus to demonstrate synthesis of amino acids from hydrogen, methane, ammonia, and water under abiotic conditions.

3. Self-Replicating Systems

a. From accumulated organic compounds emerged replicating systems consisting of DNA, RNA, and proteins.

b. Clay particles may have first served as templates to assemble amino acids into proteins with enzymatic activity.

c. Ribonucleotides may have then stuck to the clay or amino acids, and eventually replaced clay as a template.

d. How DNA entered the picture is not yet clear, but we do know that some reactions were more probable than others—not random.

4. The First Plasma Membranes

a. The metabolism in living cells cannot occur without a barrier against the chemical actions on the outside.

b. The first cells were probably membrane-bound sacs containing nucleic acids that served as templates for proteins.

c. Sidney Fox heated amino acids to form protein chains, which when allowed to cool self-assembled into small spheres that were selectively permeable.

C. Drifting Continents and Changing Seas

1. Plate tectonics refers to the arrangement of the earth’s lithosphere in slablike plates that are in motion.

2. Continents have collided and split during the earth’s history.

a. Gondwana was the continent of the Paleozoic.

b. Pangea was the later continent that extended from pole to pole, then began breaking up during the Mesozoic.

3. When the land masses separated, speciation proceeded; when the land masses collided, diversity declined.

II. The Geologic Eras and Life

A. The Archean and Proterozoic Eras

1. Archean era (3.7 to 2.5 billion years ago) was the time of macromolecule synthesis plus the origin of anaerobic and photosynthetic cells (prokaryotes).

2. In the Proterozoic era (2.5 billion to 700 million years ago), photosynthetic bacteria and eukaryotic cells (algae, fungi) were abundant, oxygen accumulated, and aerobic respiration evolved.

B. The Paleozoic Era

1. During the Cambrian period, nearly all of the major phyla evolved; most organisms lived on or near the sea floor (trilobites were a dominant group).

2. In the Ordovician period, the Gondwana continent drifted southward, shallow marine environments were formed, reef organisms flourished, and glaciers formed to trigger the first mass global extinction.

3. In the Silurian and Devonian periods, Gondwana drifted northward, reef organisms recovered, predatory fishes flourished, and amphibians and stalked plants were moving onto land.

4. In the Carboniferous period, major radiations of plants and animals occurred as land masses were alternately flooded and drained; coal deposits formed.

5. In the Permian period, insects, amphibians and reptiles flourished; formation of Pangea supercontinent caused greatest of all mass extinctions.

C. The Mesozoic Era

1. In the Triassic period, mammals originated—small and few in number.

2. In the Jurassic period, dinosaurs emerged as rulers.

3. In the Cretaceous period, flowering plants emerged, overwhelming the gymnosperms; dinosaurs became extinct.

D. The Cenozoic Era

1. Major changes in land mass configurations, climates, and adaptive zones occurred.

2. Major adaptive radiation activity set the stage for major extinctions.