CHAPTER 26 EARLY EARTH AND THE ORIGIN OF

LIFE

• The Earth formed about 4.5 billion years ago

• No clear fossils have been found in the oldest surviving Earth rocks, from 3.8 billion years ago.

• The oldest fossils that have been uncovered were embedded in rocks from western Australia that are 3.5 billion years ago.

Life on Earth originated between 3.5 and 4.0 billion years ago

• The earliest organisms were prokaryotes.• Relatively early, prokaryotes diverged into two

main evolutionary branches, the bacteria and the archaea.

Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years ago

• Photosynthesis probably evolved very early in prokaryotic history.

• Cyanobacteria, photosynthetic organisms that split water and produce O2 as a byproduct, evolved over 2.7 billion years ago.• This early oxygen initially reacted with dissolved iron

to form the precipitate iron oxide. This can be seen today in banded iron formations.

Oxygen began accumulating in the atmosphere about 2.7 billion years ago

• The first clear evidence of a eukaryote appeared about 2.1 billion years ago.

• This is the same time as the oxygen revolution that changed the Earth’s environment so dramatically.• Chloroplasts and mitochondrion evolved

Eukaryotic life began by 2.1 billion years ago

• A great range of eukaryotic unicellular forms evolved into the diversity of present-day “protists.”

Multicellular eukaryotes evolved by 1.2 billion years ago

• Recent fossils finds from China have produced a diversity of algae and animals from 570 million years ago, including preserved embryos.

• Most of the major groups of animals make their first fossil appearances during the relatively short span of the Cambrian period’s first 20 million years.

Animal diversity exploded during the early Cambrian period

• Most orders of modern mammals, including primates, appeared 50-60 million years ago.

• Humans diverged from other primates only 5 million years ago

Plants, fungi, and animals colonized the land about 500 million years ago

• Spontaneous generation- life arises from nonliving matter

• In 1862, Louis Pasteur conducted broth experiments that rejected the idea of spontaneous generation even for microbes.

• Biogenesis- all life today arises only by the reproduction of preexisting life.

The Origin of Life

• One hypothesis is that chemical and physical processes in Earth’s primordial environment eventually produced simple cells.(1) the abiotic synthesis of small organic molecules

(2) joining these small molecules into polymers

(3) origin of self-replicating molecules

(4) packaging of these molecules into “protobionts”

• 1920’s, A.I. Oparin and J.B.S. Haldane

• Conditions on the early Earth favored the synthesis of organic compounds from inorganic precursors.

• The reducing environment in the early atmosphere would have promoted the joining of simple molecules to form more complex ones.

• The energy required could be provided by lightning and the intense UV radiation that penetrated the primitive atmosphere.

1. Abiotic synthesis of organic molecules

• 1953, Stanley Miller and Harold Urey

• The atmosphere model consisted of H2O, H2, CH4, and NH3.

• It produced a variety of amino acids and other organic molecules.

• Monomers should link to form polymers without enzymes and other cellular equipment.

• Researchers have produced polymers, including polypeptides, after dripping solutions of monomers onto hot sand, clay, or rock.

2. Laboratory simulations of early-Earth conditions have produced organic polymers

• Many researchers have proposed that the first hereditary material was RNA, not DNA.• RNA can also function as an enzyme

• Short polymers of ribonucleotides can be synthesized abiotically in the laboratory.

3. RNA may have been the first genetic material

• 1980’s Thomas Cech- discovered that RNA molecules are important catalysts in modern cells.

• Ribozymes- RNA catalysts that remove introns from RNA.

• Ribozymes also help catalyze the synthesis of new RNA polymers.

• Protobionts- maintain an internal chemical environment from their surroundings and may show some properties associated with life, metabolism, and excitability.

• Liposomes- droplets of abiotically produced organic compounds that form from lipids. Forms a molecular bilayer like the lipid bilayer of a membrane.

4A. Protobionts can form by self-assembly

• If enzymes are added, they are incorporated into the droplets.

• The protobionts are then able to absorbsubstrates fromtheir surroundingsand release theproducts of thereactions catalyzedby the enzymes.

• Once primitive RNA genes and their polypeptide products were packaged within a membrane, the protobionts could have evolved as units.

• The most successful protobionts would grow and split, distributing copies of their genes to offspring.

• Later, DNA would replace RNA for storing genetic information.

4B. Natural section could refine protobionts containing hereditary information

• Laboratory simulations cannot prove that these kinds of chemical processes actually created life on the primitive Earth. They describe steps that could have happened.• Among the debates is whether organic monomers on

early Earth were synthesized in deep-sea vents or reached Earth on comets and meteorites.

• The leap from an aggregate of molecules that reproduces to even the simplest prokaryotic cell is immense, and change must have occurred in many smaller evolutionary steps.

Debates about the origin of life abounds

• 1969, R.H Whittaker, A five-kingdom system: Monera, Protista, Plantae, Fungi, and Animalia.

The five-kingdom system reflected increased knowledge of life’s diversity

• Three-domain system: Bacteria, Archaea, and Eukarya, as superkingdoms.

Arranging the diversity of life into the highest taxa is a work in progress