The Emergence of Complex Life

It is an error to imagine that evolution signifies a constant tendency to increased perfection. That process undoubtedly involves a constant remodeling of the organism in adaptation to new conditions; but it depends on the nature of those conditions whether the direction of the modifications effected shall be upward or downward

Thomas Henry Huxley (1823-1913)

The Emergence of Complex LifeWe wish to learn• What evolutionary advances have taken place at the

level of the cell?• What are the major events in the history of life?• What causes extinctions, and how are extinctions

related to opportunities for new evolutionary advances?

• Are rates of extinction and rates of evolution uniform or variable?

The Emergence of Complex Life

“Every individual alive today, the highest as well as the lowest, is derived in an unbroken line from the first and lowest forms”

- August Weismann

Life’s Origins – A Chronology

4.6 bya formation of primitive earth and atmosphere

3.8 bya first chemical evidence of life

3.5 bya first fossils of procaryotic cells

3 bya advent of photosynthesis

2.5 – 2 bya communities of procaryotes (stromatolites)

2 – 1.5 bya oxygen accumulation

1.5 bya eukaryotic algae

0.6 bya Cambrian explosion

Evolution of Earth’s Atmosphere

Life’s Origins – The Questions

• Where did the raw material for life come from?

• How did monomers develop?

• How did polymers develop?

• How did an isolated cell form?

• How did reproduction begin?

Life’s Origins – Best Answers• Where did the raw material for life come from?

– Early earth’s atmosphere

• How did monomers develop?– Miller-Urey experiment

• How did polymers develop?– Polymerization on clays, evaporation

• How did an isolated cell form?– Enclosed membrane of lipid cells

• How did reproduction begin?– RNA has the ability both to self-replicate and catayze

reactions

Oparin’s HypothesisIn the atmosphere of the early Earth, energy in the form

of ultraviolet light from the sun or lightning discharges could have created complex organic molecules from gasses such as CH4, NH3, and H2.

These complex molecules might have been similar to the building blocks of life – the amino acids which, when strung together in long chains, from proteins.

Once formed, the complex organic molecules could have somehow clumped together in larger units, eventually taking on the characteristics of primitive cells.

The gradual synthesis would have taken place in the early ocean, which he described as a “soup” of organic molecules.

Miller’s Experiment• Miller’s classic experiment produced the

organic building blocks of life from a simulated “primitive atmosphere” of methane, ammonia, and hydrogen.

• Using a high-energy electrical spark to simulate natural lightning, amino acids were formed.

• More recent experiments indicate that the ammonia and methane (though to be uncommon in the primordial atmosphere) can be replaced by carbon dioxide, which was abundant in the early atmosphere.

• Recent experiments also show that the electrical discharge mechanism can be replaced by using energy from ultraviolet light.

Findings Since Miller

• Amino acids can be replaced from other, more common pre-cursors, and using UV light instead of an electric spark

• Molecules able to catalyze chemical reactions have been formed

• RNA has been shown to have catalytic as well as self-replicating capability

• How did polymers develop?– Polymerization on clays, evaporation

• How did an isolated cell form?– Enclosed membrane of lipid cells

aggregation of macromolecules

Lipids in an aqueous solution form coacervates

Molecules of living organisms are rich in carbon compounds containing hydrogen

suggests little or no free oxygen on primitive earthOnly 20 amino acids of the left-handed variety are used by living things in proteins

suggests a single origin of lifeDNA and RNA are the universal basis of all life forms

suggests great advantage of this molecular machinery for reproduction and growth

ATP is the universal energy currency of all living thingssuggests a common origin for metabolism

In all cells, the first steps of carbohydrate metabolism involve fermentation, and the last steps in aerobic organisms use oxygen in respiration

suggests that aerobic respiration evolved from anaerobic

Molecular Clues

DNA RNA PROTEIN(Genetic (Information (Catalytic and Information)

and catalytic) Structural)

A. DNA

B. RNA

C. Protein

D. Carbohydrates

E. Lipids

Which came first?Life’s origin requires a molecule that can both store information and catalyze the synthesis of other molecules. RNA can catalyze simple reactions and can help as a template for protein synthesis and for more RNA synthesis. This suggests that RNA was probably the first genetic molecule to start life. Later we suspect that DNA evolved to be a more stable molecule, and proteins evolved to be more efficient enzymes. RNA with catalytic activity is referred to as ribozyme.

Crises and Innovation in Early Life

Heterotrophy (consuming organic compounds) almost certainly evolved before autotrophy (producing organic compounds from inorganic materials)

Innovation: autotrophy. The earliest autotrophs likely derived their H from H2 or H2S (akin to chemosynthesis by bacteria of deep sea vents)

Crisis: the H source became exhausted

Innovation: Photosynthesis (using energy of sunlight to cleave H from H20)

Crisis: the resulting O2 poisoned the atmosphere (after more than one billion years of earth ‘rusting”)

Innovation: aerobic respiration

Advent of the Eukaryotic Cell

• Prokaryotic cell- lacks internal membranes- little internal organization- bacteria, blue-green algae

• Eukaryotic cell- nucleus (internal membrane)- sub-cellular organelles

-chromosomes-mitochondria-chloroplasts

- plants, animals, protozoans, fungi

Eukaryotic and Prokaryotic Cells

The Probable Origin of Mitochondria andChloroplasts in Eukaryotic Cells

Endosymbiont origin

-accidental?

-Benefit was efficiency?

-Benefit was ability to become larger (to escape being engulfed)?

Symbiosis Within a Modern Cell• The ancestors of the chloroplasts in today's plant cells may

have resembled Chlorella, the green, photosynthetic, single-celled algae living symbiotically within the cytoplasm of the Paramecium pictured here.

Multicellularity• Size limits on how large a single cell can grow and

still function efficiently • One solution: form colonies (filamentous green

algae, sponges, etc.) Functions are not coordinated.

• Advanced multicellular organisms show cell differentiation. Specialized cells form tissues, different tissues to act collectively as organs, and different organs coordinate within the organism

• Evolved more than one billion years ago

The four eons of earth history. Bya = billion years ago, mya = million years ago

The three Eras of the Phaneozoic, further divided into periods.

Major events and mass extinctions are noted

Major Events in the History of Life

• The history of life involves enormous change• On occasion many species went extinct in a short

time – mass extinctions• Over time, life has become more diverse and more

complex• Extinction is commonplace – average species lasts

2 - 10 million years; on average, 1 –2 species go extinct per year.

• The Earth’s geological and biological histories are intertwined.

New Arrivals, Diversification, and Decline

Punctuated Equilibrium• Two views of evolutionary change

- gradual and steady, verses:

- long periods of stasis interrupted by episodes of rapid change?

• Raises key questions

- rate of evolutionary change

- nature of process• Fossil record not precise enough for definitive

answers

The Burgess Shale provides an exceptional view of life’s diversity at the beginning of the Paleozoic. Some forms survive today, others are very ancient history.

If one could rewind and re-play the tape of life, would the outcome be the same?

Causes of Extinctions

• Not just species, but families and phyla disappear• Most taxa that ever lived are extinct• Causes include:

- evolution into descendent form

- due to changes in physical environment

- due to appearance of biologically superior life forms (predaton, competition)

• These are surface answers

Mass Extinction

• “a relatively brief period of time in which more species go extinct than usual.”

• Five major ME mark end of: Ordovician, Devonian, Permian, Triassic, Cretaceous

• K – T event is best known- end of age of reptiles - 63-66 mya- asteroid evidence: iridium, crater

• Opportunity – adaptive radiation

This Iridium signal led a Berkeley physicist to propose that the impact of a huge meteor some 60-65 mya caused the K-T extinction event and the extinctions of the dinosaurs.

Clocks in Molecules

Evolution and Natural Selection• The history of life involves

enormous change• Over time, life has become

more diverse and more complex

• Extinction is commonplace• The Earth’s geological and

biological histories are intertwined