The Origin of Life

Chapter 26

The History of Life on Earth

Spontaneous Generation

–Concept stating that life generates from other things unlike itself

• Ex: rotting meat gives rise to maggots and then to flies

Francesco Redi (1668)

–2 jars with rotting meat; 1 open to the air, the other covered with gauze

Lazzaro Spallanzani (1700’s)

–2 Flasks with gravy, both boiled. One sealed, the other open to the air

Louis Pasteur

Father of Microbiology and its effect on life

In 1862, he too used a broth and boiled the substance

So, if it has been shown that life must come from pre-existing life (biogenesis), then which came first, the chicken or the egg? Where/when/how did the first life appear on Earth?

One credible hypothesis is that chemical and physical processes in Earth’s primordial environment eventually produced simple cells

It’s important to understand that no one knows exactly how life arose on EarthJust like any investigator (Ex: CSI), you must start with one piece of evidence and try to explain it–That means be able to replicate in

the lab how that piece of evidence came to be

Then, when enough experimentally supported pieces of evidence have been gathered, an all-encompassing conclusion can be drawn (theory)

Under one hypothetical scenario, this occurred in four stages:

(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”

Abiotic synthesis of small O-molecules

Origin of the universe

–Big Bang – lighter elements (mostly hydrogen)

–Stars (fusion) – up to Carbon

–Super Novae – heavier elements

Origin of Earth

–Crust solidified

–Volcanoes spewed inorganics, creating early atmosphere

Abiotic synthesis of small O-molecules

Earth’s first atmosphere most likely contained: CO, CO2, H2, and H20 mixed with some N2 and possibly other gases such as ammonia (NH3) and methane (CH4)

–All inorganic molecules

Abiotic synthesis of small O-molecules

What is missing?

–Little or no O2. Why not?

• No photosynthetic organisms to produce O2

–O2 binds easily to other compounds – it doesn’t stay O2 for very long

• Ex: CO2, H2O

Abiotic synthesis of small O-molecules

In the 1920’s, A.I. Oparin and J.B.S. Haldane independently proposed idea

Earth’s early atmosphere was much different that today; conditions could have been conducive to the formation of simple organic materials

Abiotic synthesis of small O-molecules

In 1953, American scientist Stanley Miller tested Oparin’s hypothesis by recreating Earth’s early environment with all of the inorganic molecules

He then exposed the environment to electric sparks (simulated lightning)

Abiotic synthesis of small O-molecules

In a few days, organic molecules started to form

Every run of the experiment provided amino acids, ATP, and Adenine

Abiotic synthesis of small O-molecules

Alternate sites proposed for the synthesis of organic molecules include submerged volcanoes and deep-sea vents where hot water and minerals gush directly into the deep, cool ocean

Abiotic synthesis of small O-molecules

Another possible source for organic monomers on Earth is from space, including via meteorites containing organic molecules that crashed to Earth

–Panspermia

From monomers to polymers

With constant energy sources and enough time (millions/billions of years), the newly born Earth’s oceans would have been teeming with simple O-molecules

These monomers just needed a way to combine to become polymers

Clay theory: clay acted as a template from which O-molecules replicated themselves

–Dissolved O-molecules splash on hot sand, clay, or lava (or around deep sea vents)

–Water evaporates, leaving the O-molecules behind

–UV radiation and iron pyrite catalyze

From monomers to polymers

From monomers to polymers

Self-replicating molecules

DNA, RNA, or Protein first?

Combination?

Many believe the first hereditary material was RNA, not DNA–RNA can also function as enzymes

Self-replicating molecules

Short RNA polymers can be synthesized abiotically in the lab–If these polymers are added to a

solution of ribonucleotide monomers, sequences up to 10 bases long are copied

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

Self-replicating molecules

In the 1980’s Thomas Cech discovered RNA molecules are important catalysts in modern cells

RNA catalysts (ribozymes) remove introns from RNA

Ribozymes also help catalyze the synthesis of new RNA polymers

In the pre-biotic world, RNA molecules may have been fully capable of ribozyme-catalyzed replication

Self-replicating molecules

Because RNA is only single stranded, its conformation can be quite different than DNA, based upon the nucleotide sequence

Varying conformations of RNA strands allows natural selection to favor some strands and “weed out” others

Occasional copying errors lead to mutations – the source of variation

Self-replicating moleculesRNA-directed protein synthesis may have begun as weak binding of specific amino acids to bases along RNA molecules, which functioned as simple templates holding a few amino acids together long enough for them to be linked– This is one function of rRNA today in

ribosomesIf RNA synthesized a short polypeptide that behaved as an enzyme helping RNA replication, then early chemical dynamics would include molecular cooperation as well as competition

Self-replicating moleculesEventually, an RNA template would have helped synthesize a single strand of DNA, which would have quickly made its complementary strand

DNA is a more stable molecule

– If it was synthesized based upon an RNA code, it could still produce RNA replicas

Road to “Protobionts”

Protobionts are groups of abiotically produced molecules

–Maintain separate internal environment

–“Reproduce”

–May contain required materials for some chemical rxns

• i.e., they exhibit some attributes of living things

Road to “Protobionts”

Amphipathic lipids to form bilayers, which can wrap to form spheres

–Can grow or shrink due to osmosis when placed in different salt concentrations

–Can store E as a membrane potential

–Can “eat” (engulf) smaller spheres

Road to “Protobionts”Membranes separate internal from external environmentsProvides stability and compartmentalizationIf one metabolic process generates E, a membrane can keep the E for itself (nat. sel.)Protobiont can evolve as a unit

Earth's HistoryProjected on a 24-hour Day

Formation of Earth

First Earth rocks

12 12

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a.m. 6

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6 p.m.

First prokaryotes

First atmospheric oxygen

First eukaryotes

First multicellular organisms

First flowers

First humans(11:59:40)

First humans(11:59:40)

Billions ofyears ago

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Diversity of Life

Simple cells, with genetic info, that could replicate now found on Earth

Mutations driving force behind nat. sel.

Diversity of Life

Geology dictated what life could evolve–Pangea allowed mixing of gene

pools–Breakup of Pangea isolated

populations

Life dictated what life could evolve

–Lack of O2 drove anaerobic resp.

–Photosynthesizers put O2 in air, driving evolution of aerobic resp.

Origins of Organelles

Because of the environment, heterotrophic life could have lived off this organic mix for some time

If one organism engulfed another to eat it, but the prey turned out to benefit the predator, a mutualism could be formed (Endosymbiont Theory)

Heterotrophic eukaryotes could have formed

Anaerobic, predatoryprokaryotic cell

engulfsan aerobic bacterium

Aerobic bacterium

Descendents of engulfed bacterium

evolve into mitochondria

Origins of Organelles

But Natural Selection would have favored organisms that could harness an outside E source to survive

At some point, an ancient form of photosynthesis evolved

The first autotrophs were very successful and spread throughout the environment

Descendents of photosynthetic

bacteria evolve into chloroplasts

Photosynthetic bacterium

Mitochondria-containing cell engulfs

photosynthetic bacteria

ParameciumParamecium sp. sp.

ChlorellaChlorella sp, sp,a green a green algaealgae

Origins of DiversityEarth formed ~4.5 Bya

Earth’s crust didn’t form until ~4Bya

Oldest fossils found formed ~3.5Bya

So, life had to have originated sometime between ~4-3.5Bya

– Crust, cooler temps, liquid water

– The life resembled bacteria

Origins of DiversityProkaryotes dominated from ~3.5-2Bya

Stromatolites are sources of prokaryotic fossils

– Cyanobacteria that lived in huge floating mats

– They’d deposit CaCO3, which left layered effect

– Probably responsible for Earth’s O2

atmosphere

Origins of DiversityMost of the O2 liberated from H2O probably reacted with Fe to form iron oxide

Seen in many “rusted” banded patterns

~2.7Bya, enough O2 was being formed to change the atmospheric compositions

Origins of DiversityO2 oxidizes so much that most of the existing prokaryotic life died off

Others evolved mechanisms to utilize O2

First eukaryotic cells formed ~2.7-2.1Bya – right about the time O2 was becoming dominant

This “coincidence” could help explain how aerobic respiration evolved (environmental factors putting pressures on the organisms)

Origins of DiversityMulticellular organisms appear ~1.5-1.2Bya

Most cnidarians and poriferans were present in late Precambrian

The “Cambrian Explosion” is where the real animal diversity that we see today came from

~550-510Mya

Could be due to a global “thawing” period

Origins of DiversityLand invasion took place ~500Mya

Organisms had to evolve ways to prevent water loss

Plants helped “bring” animals to land by providing food sources

Herbivores “brought” their predators to land

Terrestrial vertebrates, tetrapods, evolved from fishes

Most modern mammals appeared ~60-50Mya

Hominids diverged only ~5Mya

Write one paragraph explaining the significance of this cartoon