CHAPTER 26 Early Earth and the Origin of Life

Time line: Introduction to the History of Life1. Life on Earth originated between 3.5 and 4.0 billion years ago

2. Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years

ago

3. Oxygen began accumulating in the atmosphere about 2.7 billion years ago

4. Eukaryote life began by 2.1 billion years ago

5. Multicellular eukaryotes evolved by 1.2 billion years ago

6. Animal diversity exploded during the early Cambrian period

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

Figure 26.0 A painting of early Earth showing volcanic activity and photosynthetic prokaryotes in dense mats

Figure 26 Volcanic activity and lightning associated with the birth of the island of

Surtsey near Iceland; terrestrial life began colonizing Surtsey soon after its birth

Phylogenetic trees: Timeline and order of events that shaped evolution of life. Branch points show derived characteristics unique to each group/clade.

Fig. 26.1

Alternatively, we can view these episodes with a clock analogy.

Fig. 26.2

Origin of life (4 billion - 3.5 billion years ago)

Earth formation - 4.5 billion years ago - hostile to life - asteroids, volcanoes, water vaporized

Oldest fossils - Australia - 3.5 billion years old (bacteria like); evidence of earth cooling to a temp. when liquid water persisted.

First living organisms - prokaryotes (3.5 - 2 million

years ago)

Proof: Prokaryote fossils - stromatolites (fossilized layered microbial mats) and sediments from ancient hydrothermal vent (deep ocean vents) habitats.

Figure 26.4 Bacterial mats and stromatolites

Figure 26.4x Stromatolites in Northern Canada

Early life did not use light/split water - they were chemoautotrophs

Energy for synthesis of organic compounds came from chemicals - iron, H2S, NH3

H2S ---------> 2H+ + S

Photosynthesis probably evolved very early in prokaryotic history.

Oxygen began accumulating 2.7 billion years ago

Cyanobacteria, photosynthetic organisms that split water and produce O2 as a byproduct, evolved over 2.7 billion years ago.

Figure 26.5 Banded iron formations are evidence of the vintage of oxygenic photosynthesis

Cyanobacteria

This “corrosive” O2 had an enormous impact on life, dooming many prokaryote groups. (OXYGEN KILLS!) Some species survived by remaining

anaerobic.

Others evolved mechanisms to use O2 in cellular respiration - DO or DIE!

Eukaryotic cells are generally larger and more complex than prokaryotic cells.

This is due to the “endosymbiotic prokaryotes” that evolved into mitochondria and chloroplasts - major event in eukaryotic evolution

Eukaryotic life began by 2.1 billion years ago

Multicellular organisms, differentiated from a single-celled precursor.

Snowball hypothesis - earth covered in ice from 750 to 570 million years ago; prevented eukaryotic diversification in that period

Life stayed in parts where ice did not reach - deep sea vents/hot springs

Late Precambrian/Early Cambrian - diversification of eukaryotes started (thawing of snowball!)

Multicellular eukaryotes evolved by 1.2 billion years

ago

Fig. 26.6

China - beautifully preserved animal embryos.

Fig. 26.7

Animal diversity exploded during Cambrian period

Colonization of land - important event in evolution - plants (with fungi) moved onto land and animals followed Cyanobacteria and other photosynthetic

prokaryotes coated damp terrestrial surfaces well over a billion years ago.

Terrestrial adaptations for land dwelling (later)

Plants, fungi, and animals colonized the land about 500

million years ago

Land Plants ----> herbivores ---> carnivores

Most diverse award: Arthropods (including insects and spiders) and certain vertebrates (including amphibians, reptiles, birds, and mammals).

Modern mammals, including primates, appeared 50-60 million years ago.

Humans diverged from other primates only 5 million years ago

TETRAPODS

Nonliving materials became ordered into aggregates that were capable of self-replication and metabolism.

Greeks - spontaneous generation (life from nonlife); not correct!

Evolution of the first cells: chicken or egg problem

1862, Louis Pasteur - experimentsthat rejected spontaneousgeneration even for microbes.

A sterile brothwould “spoil” onlyif microorganismscould invade fromthe environment.

Fig. 26.9

NOW - Principle of biogenesis - life from life.

Early earth - different conditions - remember NO OXYGEN to attack complex aggregates of molecules + Energy sources, such as lightning, volcanic activity, and ultraviolet sunlight, were more intense than what we experience today.

SOOOOO, spontaneous generation was possible in early earth only

Evolution of first cell: Four stages:(1) The abiotic synthesis of small organic molecules;

(2) The joining these small molecules into polymers:

(3) The origin of self-replicating molecules;

(4) The packaging of these molecules into “protobionts.”

This hypothesis can be tested in the laboratory.

Stanely Miller - simulated conditions on early Earth (no oxygen, reducing environment with inorganic gases like H2, CO2, NH3, CH4; lightning/UV with no ozone. Favored the synthesis of organic compounds from inorganic precursors.

1) Abiotic synthesis of organic monomers

The Miller-Urey experiments produced a variety of amino acids and other organic molecules.

Abiotic origin hypothesis - 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 simulation of early-Earth conditions produced organic

polymers

Life = reproduction = inheritance.

First hereditary material was RNA, not DNA. Because RNA can

also function as an enzyme, it helps resolve the paradox of which came first, genes or enzymes.

3) RNA may have been the first genetic material

(important)

3) SELF-REPLICATING RNA: Short polymers of ribonucleotides can be synthesized abiotically in the laboratory. If these polymers are added to a solution of ribonucleotide

monomers, sequences up to 10 bases long are copied from the template according to the base-pairing rules.

If zinc is added, the copied sequences may reach 40 nucleotides with less than 1% error.

RNA is acting as a self-replicating enzyme - Ribozyme! (remember sNRNPs in spliceosome!!)

Fig. 26.11

3) Natural selection of the ‘fittest RNA’:

RNA sequences evolve in abiotic conditions.

RNA molecules - genotype (nucleotide sequence) - phenotype (three-dimensional shape) that interacts with surrounding molecules.

Some RNA sequences are more stable and replicate faster and with fewer errors - selected

RNA-directed protein synthesis - weak binding of specific amino acids to bases along RNA molecules, (rRNA today in ribosomes).

If RNA synthesized a short polypeptide that behaved as an enzyme ahhhhh…

Protobionts - aggregates of abiotically produced molecules (RNA and macromolecules) enclosed in a membrane (separate internal environment).

Metabolism and excitability - properties of life.

4) Protobionts can form by self-assembly

QuickTime™ and aPhoto - JPEG decompressor

are needed to see this picture.

Liposomes behave dynamically, growing by engulfing smaller liposomes or “giving birth” to smaller liposomes.

Fig. 26.12a

Most successful protobiont - reproduces and passes on its traits: RNA genes and their polypeptide products; eventually RNA replaced by DNA.

4)Natural selection could refine protobionts containing hereditary

information

Fig. 26.13

Past - Kingdom was the highest taxonomic category. NOW - DOMAIN!

Old system - only two kingdoms of life - animal or plant. Bacteria, with rigid cell walls, were placed with

plants. Even fungi, not photosynthetic and sharing

little with green plants, were considered in the plant kingdom.

Photosynthetic, mobile microbes were claimed by both botanists and zoologists.

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

diversity

In 1969, R.H Whittaker argued for a five-kingdom system: Monera, Protista, Plantae, Fungi, and Animalia.

Fig. 26.15

Recognizes Prokaryotes and eukaryotes

Plants - autotrophs; Fungi- decomposers/extracelluar digestion; Animals - heterotrophs with digestive cavities

Problems with 5 Kingdom system: Protista consisted of all eukaryotes that

did not fit the definition of plants, fungi, or animals - it was not monophyletic. Most protists are unicellular. However, some multicellular organisms, such

as seaweeds, were included in the Protista because of their relationships to specific unicellular protists.

The five-kingdom system prevailed in biology for over 20 years.

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

3 Domain System

A second challenge to the five kingdom system comes from systematists who are sorting out the phylogeny of the former members of the kingdom Protista. Molecular systematics and cladistics have

shown that the Protista is not monophyletic. Some of these organisms have been split

among five or more new kingdoms. Others have been assigned to the Plantae,

Fungi, or Animalia.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Clearly, taxonomy at the highest level is a work in progress. It may seem ironic that systematists are

generally more confident in their groupings of species into lower tax than they are about evolutionary relationships among the major groups of organisms.

Tracing phylogeny at the kingdom level takes us back to the evolutionary branching that occurred in Precambrian seas a billion or more years ago.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

There will be much more research before there is anything close to a new consensus for how the three domains of life are related and how many kingdoms there are. New data will undoubtedly lead to further

taxonomic modeling. Keep in mind that phylogenetic trees and

taxonomic groupings are hypotheses that fit the best available data.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings