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

The Beginning

• Emergence of life from non-life- (Oparin and Miller experiments)

• Respiration – energy from organic matter• Self-replication• First cell (3.5 bya) • Accumulation of oxygen in atmosphere and

aerobic respiration – 1 bya• Aerobic respiration and ozone layer

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.

Crises and Innovation in Early LifeHeterotrophy (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 chemosythesis 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

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.

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

New Arrivals, Diversification, and Decline

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 (pred, comp)

• 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.

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

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