BISC 121 - The Origin and History of Life on Earth - Midterm 3

When did life on Earth originate?

Between 3.5 and 4 billion years ago

When did prokaryotes dominate evolutionary history?

From 3.5 to 2 billions years ago

When did oxygen begin accumulating in the atmosphere?

About 2.7 billion years ago

When did eukaryote life begin?

2.1 billion years ago

When did multicellular eukaryotes evolve?

1.2 billion years ago

What does a time line or phylogenetic tree allow one to do?

View the chronology of the major episodes that shaped life

What is the traditional geological record studied by?

Fossils

What are the state of fossil records in Earth’s history?

Most of the Earth’s history has poor fossil records due to extreme age, and must be inferred from incomplete evidence

What parts of the geological record are most of the Earth’s history in?

Archaean and Hadean

What makes the geological record easier to understand?

A clock analogy

What were Earth’s only organisms like for the first three-quarters of evolutionary history?

Microscopic and mostly unicellular

When did the Earth form?

4.5 billion years ago

Why was it unlikely that life could survive for the first few hundred million years of Earth’s existence?

Rocks left over from the origin of the solar system bombarded the surface and it was very hot

When are Earth’s oldest surviving rocks from? Have any clear fossils been found in them?

3.8 billion years ago, no clear fossils have been found in them

What do we mean when we say a rock is 3.8 billion years old?

The rock had no significant changes until 3.8 billion years ago

Where were the oldest fossils uncovered? How old are they?

Western Australia, 3.5 billion years ago

What did the presence of the oldest fossils resemble and what do they imply?

They resemble bacteria and imply that life originated much earlier

What is a possibility for when life originated?

3.9 billions years ago, when Earth began to cool to a temperature at which liquid water could exist

What is a characteristic of prokaryotes?

Cells without membrane-bound nuclei

What are stormatolites?

Fossilized layered microbial mats, one of two rich sources for early prokaryote fossils

What is the second rich source for early prokaryote fossils?

Sediments from ancient hydrothermal vents

What do the two rich sources for early prokaryote fossils indicate about the metabolism of prokaryotes?

It was already diverse 3 billion years ago

When did photosynthesis probably evolve?

Very early in prokaryotic history

What does it mean when all photosynthesis requires reducing power?

All photosynthesis needs a source of available hydrogen atoms because CO2 needs to be reduced to make organic carbon

What was the metabolism of early versions of photosynthesis like?

Photosynthesis did not split water and liberate oxygen while using the H atoms. Organisms relied on rare compounds like H2S for reducing power. 

What is cyanobacteria? When did it evolve? Why did it become very successful?

Cyanobacteria are photosynthetic organisms that split water and produce O2 as a byproduct. It evolved over 2.7 billion years ago. Cyanobacteria became very successful because water is readily available. 

How do we know when cyanobacteria evolved?

The early oxygen initially reacted with dissolved iron to form the precipitate iron oxide, which can be seen today in banded iron formations.

What happened to terrestrial rocks with iron when oxygen began accumulating in the atomsphere?

They began oxidizing

Describe the accumulation of oxygen

It was gradual between 2.7 and 2.2 billion years ago. It shot up to 10% of current values shortly afterward. 

What impact did the reactive and mostly toxic gas O2 have on life?

It doomed many prokaryote groups. This was probably the world’s biggest air pollution event. 

How did some species survive the reactive/toxic O2? What are their descendants called?

Some didn’t adapt because they were living in anaerobic environments. ”Strict anaerobes”

How did some species evolve against reactive/toxic O2?

They evolved protective mechanisms against O2 and then evolved the ability to use O2 in cellular respiration, which uses oxygen to help harvest the energy stored in organic molecules. 

When are the first clear eukaryote fossils from? What does other evidence place it as?

2.1 billion years ago. Other evidence says eukaryotes originated as early as 2.7 billion years ago

Why did the early eukaryotes originate at the same time as the oxygen revolution that changed the Earth’s environment so dramatically?

The evolution of chloroplasts from cyanobacteria. And the mitochondrion from alpha proteobacteria turned the accumulating O2 to metabolic advantage through cellular respiration. 

What did a great range of eukaryotic unicellular forms evolve into?

The diversity of present-day protists

When did multicellular organisms originate according to molecular clock estimates?

1.5 billion years ago

What is the Snowball/Slushball Earth hypothesis

Periods of nearly global ice cover from 750 to 635 million years ago may be responsible for the limited diversity and distribution of multicellular eukaryotes until the very late Precambrian

What would most life have been confined to during the Snowball Earth era

Hot springs, deep-sea vents, tropical mountaintops, locations where enough ice melted for sunlight to penetrate the surface waters of the sea

According to the snowball earth theory, what does the first major diversification of multicellular eukaryotic organisms correspond to?

The time of thawing of snowball Earth (Cambrian)

What are the scientific theories on the origin of life?

1) The first cells may have originated chemical evolution on a young Earth

2) Abiotic synthesis of organic molecules is a testable hypothesis

3) Lab simulations of early-Earth conditions have produced organic polymers

4) RNA may have been the first genetic material

5) Protobionts can form by self-assembly

6) Natural selection could refine protobionts containing hereditary info

7) Debate about the origin of life continues

Describe the theory that the first cells may have originated by chemical evolution on a young Earth

Life developed from nonliving materials that became ordered into aggregates that were capable of self-replication and metabolism.

Explain the spontaneous generation idea

That life could arise from nonliving matter, believed from the time of Greeks until the 19th century. Now a rejected idea, but people still thought it was an explanation for the rapid growth of microorganisms in spoiled foods

Describe the 1862 experiment of Louis Pasteur

Conducted broth experiments that rejected the idea of spontaneous generation even for microbes. A sterile broth would only spoil if microorganisms could invade from the environment.

All life today arises only by

Biogenesis (the reproduction of preexisting life)

What were conditions on the early earth like?

Little or no oxygen, more intense energy from UV, volcanoes

What are the four stages the origin of life may have occured in?

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

What did A.I. Oparin and J.B.S. Haldane do in the 1920s?

They independently postulated that 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

Where could the considerable energy required to make organic molecules be provided from?

Lightning and the intense UV radiation that penetrated the primitive

How was the early Sun different?

Emits more UV radiation and the lack of an ozone layer (because no free oxygen) in the early atmosphere would have allowed this radiation to reach the Earth

1952 Stanley Miller and Harold Urey experiment

Tested Oparin-Haldane hypothesis by creating in the lab, the conditions that had been postulated for early Earth. They discharged sparks in an “atmosphere” of gases and water vapor. They produced a variety of amino acids and other organic molecules. The artificial atmosphere produced consisted of H2O, H2, CH4, and NH3, probably a more strongly reducing environment than is currently believed to represent early earth. 

What alternate sites were proposed for the synthesis of organic molecules?

Submerged volcanoes and deep-sea vents where hot water and minerals gush into the deep ocean

What is another possible source for organic monomers on Earth?

Space, including via meteorites containing organic molecules that crashed to Earth

What is the abiotic origin hypothesis?

Predicts that monomers should link to form polymers without enzymes and other cellular equipment

How have researchers produced polymers and polypeptides? What does this mimic?

Dripping solutions of monomers onto hot sand, clay, or rock, This mimics when dilute solutions of monomers splashed onto fresh lava or at deep sea vents on early Earth.

What is the RNA World hypothesis

RNA may have been the first genetic material

What are the two things life needs?

Inheritance and metabolism

What helps resolve the paradox of which came first: genes or enzymes?

RNA was the first hereditary material. It can function as an enzyme as well. With RNA, the same molecule can have both functions

How can short polymers of ribonucleotides be copied?

Abiotically in simple solutions. If these polymers are added to a solution of ribonucleotide monomers, sequences up to 40 bases long are copied from the template according to the base-pairing rules, with 1% error

What did the 1980’s Nobel laureate Thomas Cech discover?

RNA molecules are important catalysts in modern cells

What are ribozymes?

RNA catalysts that remove introns from RNA. They help catalyze the synthesis of proteins. Central process in all life on Earth

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

Ribozyme-catalyzed replication

Lab experiments have demonstrated that RNA sequences can evolve where?

Abiotic conditions

Characteristics of RNA molecules

They have genotype (nucleotide sequence) and a phenotype (3D shape) that interacts with surrounding molecules

In the right conditions, some RNA sequences can

be more stable and replicate faster with fewer errors

What do occasional copying errors in RNA sequences create?

Mutations and selection screens these mutations for the most stable or best at self-replication

How may have RNA-directed protein synthesis begin?

There was a 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 ribosomes.

What came before living cells? What are these?

Protobionts: aggregates of abiotically produced molecules

What is the nature of how protobionts reproduce?

They do not reproduce precisely, but to maintain an internal chemical environment separate from their surroundings and show some properties associated with life, metabolism, and excitability

What are liposomes?

Droplets of abiotically produced organic compounds that form when lipids are included in the mix 

What do lipids form at the liposome surface?

A molecular bilayer similar to the lipid bilayer of a membrane

How do liposomes change in different salt concentrations?

They can undergo osmotic swelling or shrinking in different salt concentrations

How can liposomes store energy?

As a membrane potential, a voltage cross the surface

How do liposomes behave?

Dynamically. Grow by engulfing smaller liposomes or giving birth to smaller liposomes

What happens if enzymes are included among the ingredients for liposomes?

They are incorporated into the droplets, which could be considered protobionts

How do artificial protobionts differ from those formed in the ancient seas?

They (ancient sea protobionts) would not have possessed refined enzymes, but there could well have been protobionts that had a rudimentary metabolism that allowed them to modify substances that they took in across membranes

When could protobionts have evolved as units?

Once primitive RNA genes and their polypeptide products were packaged within a membrane

Why could molecular cooperation be refined?

Favorable components were concentrated together, rather than spread throughout the surroundings

How would the most successful protobionts behave?

They would grow and split, distributing copies of their genes to offspring. Even if only one arose initially by abiotic processes, its descendants would vary because of mutation, errors in copying RNA

Even if the odds of life originating seem impossible

It only takes one with the right stuff to start life

What refined primitive metabolism and inheritance?

Evolution via differential reproductive success of varied individuals

What was one eventual refinement of metabolism and inheritance?

Replacement of RNA as the repository of genetic info by DNA, a more stable molecule

What happened for RNA molecules after DNA appeared?

They would have begun to take on one of their modern roles - as intermediates in translation of genetic programs. Exception: RNA is the genetic material of many viruses

How does debate about the origin of life continue? (even setting aside religion)

Lab simulations can’t say what actually created life, only the steps that could have happened. Debate: whether organic monomers were synthesized on the planetary surface or were synthesized in space and reached the planet on comets and metorites - or significant contributions from both

Is there evidence or proof that life started here?

There is evidence but no proof.

Is there definitive evidence of extraterrestrial life?

No

Various moons life origin hypothesis

This moon of jupiter has a presence of ice, which led to hypothesis that liquid water lies beneath the surface over a rocky base, and may support life. Jovian moons Ganymede and Callisto are also likely to host subsurface oceans. Saturn’s moon Enceladus has a liquid water ocean beneath an icy shell, with hydrothermal vents. 

Mars life origin hypothesis

While mars is cold, dry at the surface, and lifeless today, it was probably relatively warmer, had more water, and with a CO2-rich atmosphere billions of years ago. Many scientists see Mars as an ideal place to test hypothesis about Earth’s prebiotic chemistry

Describe the leap from an aggregate of molecules that reproduces to even the simplest prokaryotic cell

Immense, and the change must have occurred in many smaller evolutionary steps

When were prokaryotes already flourishing on Earth? What does evidence say about them?

3.5 billion years ago. Very strong evidence in molecular sequences, structures, and fossils say all the lineages of life here arose from ancient prokaryotes

Arranging the diversity of life intro the highest taxa is still a work in progress, but what are the three domains (superkingdoms)?

Bacteria, Archean, and Eukarya. Pioneered by Carl Woese

What were kingdoms traditionally regarded as?

The highest taxonomic category

What were the original two kingdoms

Animal or plant

What was the 1969 R.H Whittaker five-kingdom system?

Monera, Protista, Plantae, Fungi, and Animalia

What were the Protista in Whittaker’s kingdom system?

all eukaryotes that did not fit the definition of plants, fungi, or animals

Issues with the five-kindom system

There is strong evidence that there are two distinct lineages of prokaryotes - as different from each other as are plants and animals

What is cladistic analysis?

Analysis based on molecular sequence data

How many major divisions are there in the bacteria kingdoms, currently?

More than 100

What is the explosion of discovery from?

New molecular genetic methods that do not require cultivating the organisms. New methods enable to sequencing of 16S rRNA without the need to culture the organisms.

Early 21st century simplified Tree of Life is based on what?

Small subunit rRNA (16S and 18S rRNA)