Conditions on early Earth made the origin of life possible

Synthesis of Organic Compounds on Early Earth

• Earth formed about 4.6 billion years ago (bya)• The Oparin-Haldane hypothesis stated that Earth’s early atmosphere was a reducing (e- adding) environment. Organic molecules could be synthesized from inorganic molecules or small molecules and the energy required for this synthesis could come from lightning and UV light.• Stanley Miller and Harold Urey’s experiment in 1953 demonstrated that organic molecules like amino acids could be synthesized from inorganic molecules like those found in the early Earth.

Early Atmosphere(Notice what is NOT on this list)

• Water vapor (condensed into oceans as the earth cooled)• Nitrogen and nitrogen oxides• Carbon dioxide• Methane• Ammonia• Hydrogen• Hydrogen sulfide• Originally thought to be a reducing atmosphere• http://www.chem.duke.edu/~jds/cruise_chem/Exobiology/PB

earth.html

Miller and Urey’s ExperimentELECTRICITY!!!

Organic molecules like amino acids

The precambrian world might have looked like this – 3 billion years ago

Are molecules like amino acids sufficient for the emergence of life??

• Amino acid polymers have been found to be synthesized without enzymes. Clay and sand (abiotic) have been shown to “grow” these polymers.

• What about replication and metabolism? Hereditary material?

It’s an RNA world, baby!

• Ribozymes are RNA catalysts

Science 22 September 2006:Vol. 313. no. 5794, p. 1700

Nucleotides arranging them on clay

http://exploringorigins.org/nucleicacids.html

RNA worldRNA world

• The first genetic material was probably self-replicating, catalytic RNA not DNA;

• In “RNA world”, RNA could have provided the template on which DNA was assembled

• Once DNA appeared “RNA world” gave way to “DNA world”

• The first organisms were not photosynthetic; they were probably heterotrophic

Protobionts, collections of abiotically produced molecules surrounded by a membrane-like structures

Liposomes can form when lipids or other organic molecules are added to water.

- Have a bilayer- Can undergo osmosis- Can “reproduce”

Protocell (Protobiont)

Fatty acid membrane with ribozymes inside

Present

Dimetrodon

Coccosteus cuspidatus

Fossilizedstromatolite

Stromatolites Tappania, aunicellulareukaryote

Dickinsoniacostata

Hallucigenia

Casts ofammonites

Rhomaleosaurus victor, a plesiosaur

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The Fossil Record

documents the history of life

• Keep in mind that it is an incomplete chronicle or evolutionary change. WHY?

• The fossil record is biased in favor of species that existed for a long period of time, were abundant and wide-spread, and had hard shells

• The order of fossils in rock strata tells us the sequence in which the fossils were laid down – their relative ages – it does not tell us their absolute ages.

• SO…. We use radiometric dating to determine actual age of fossils.

How do we determine the age of fossils?

Radiometric Dating – each isotope has a fixed rate of decay

Radiometric dating• Fossils contain isotopes of elements that

accumulated in the organisms when they were alive.

• For example: C-12 (most common isotope) and C-14 are both found in living organisms. When an organism dies it stops accumulating this carbon and the C-12 levels do not change over time. BUT – the C-14 isotope begins to decay into N-14.

• By measuring the ratio of carbon-14 to carbon-12, we can determine a fossil’s age

Table 25-1a

Table 25-1b

Three-domain systemExtremophilesProkaryotes Eukaryotes

Look at how this evolution happened!

Cyanobacterium - Anabaena

Early cells were heterotrophs – but this probably added extreme selective pressure on these cells due to famine.

This selected for cells that could produce their own food - autotrophs

When photosynthesis is based around water, it produces a significant by-product: oxygen. Since oxygen was highly toxic to the cyanobacteria producing it, they were forced to evolve means of protecting themselves from it, primarily by excreting it as a gas.

•Their success in this led to the steady but gradual pumping of oxygen into the Earth's atmosphere.

Cyanobacteria converted the reducing environment

of early Earth into an oxidizing one

•Cyanobacteria are the only O2-releasing prokaryotes•Atmospheric O2 is generated by photosynthetic organisms in the water-splitting step

Cross section of microbial mats showing layers of pigmented bacteria

Some bacterial mats form rock-like structures called stromatolites

Shark Bay, Western Australia

These stromatolites in Morocco formed at the bottom of a sea about 600 million years ago

This precambrian stromatolite from the Ozarks is over 1.5 billion years old

See the layer of cyanobacteria (photosynthetic) near the top of the image

This stromatolite is about 3.5 million years old

Three-domain systemExtremophilesProkaryotes Eukaryotes

Look at how this evolution happened!

Early prokaryotes may have arisen near hydrothermal vents

http://www.bact.wisc.edu/themicrobialworld/homepage.html

Hydrothermal vents are rich in sulphur and iron-containing compounds needed for ATP synthesis. Temperatures can reach 120 C.

Hot springs in Yellowstone National Park – pigmented bacterial mats

http://www.bact.wisc.edu/themicrobialworld/origins.html

Three-domain systemExtremophilesProkaryotes Eukaryotes

Look at how this evolution happened!

Eukaryotic cells arose from symbiotic interactions about 2.1 billion years ago

Mitochondria and chloroplasts have their own DNA

An anaerobic cell that contained an aerobe would have an advantage.

What do these two types of cells become millions of years later?

Evidence for endosymbiosis

• Membranes• DNA• Reproduction• http://evolution.berkeley.edu/evolibrary/artic

le/_0_0/endosymbiosis_04

Origin of Multicellularity• These early organisms lived 1.5 billion years ago.• “Snowball Earth Hypothesis” states that ice ages

caused glaciers to cover most of the surface of the Earth and that life would have been limited to the deep sea or hot springs or equatorial regions without ice cover.

• When “snowball Earth” thawed roughly 565 mya, the fossil record shows a rapid diversification of life

The first multicellular organisms were colonies, collections of autonomously replicating cells.

Cambrian – large changes in seawater composition occurred in this period, e.g.extreme fluctuations in carbon, sulphur, and strontium

During this period extremely varied complex animal forms existed.

Cambrian Explosion – 535-525 mya• Pre-cambrian:

– Cnideria, sponges and molluscs – Herbivores, filter-feeders or scavengers, not hunters– Soft-bodied

• Cambrian Explosion:– Short period of time (10 my)– Predators became prevalent – teeth, claws– Defensive adaptations – sharp spines, body armor– Larger organisms

(a) Two-cell stage 150 µm 200 µm(b) Later stage

Colonization of Land• Cyanobacteria and other photosynthetic

prokaryotes coated damp terrestrial surfaces well over 1 billion years ago.

• Fungi, plants, and animals, did not begin to colonize land until about 500 million years ago.– Plants evolution – vascular system, waxy cuticle,

reproduction– Mutualistic associations between fungi and plants

developed during this time– Arthropods rule! – Tetrapods

Earth’s major crustal plates are still changing constantly – they are floating on the hot molten underlying mantle

Not shown: A new plate is forming between East Africa and the rest of the continent

San Andreas fault east of San Luis Obispo

The earth is still changing

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Bora bora

Icelandic island at the border between the Eurasian and the North American plates, which are still moving apart.

Volcanoes shape the earth

Active volcano in Hawaii

Pangaea had split into Laurasia and Gondwana by the middle of the Mesozoic era (135 mya)

Fish and large land-dwelling reptiles existed at this time

Lungfish - a relative of early tetrapod fish first appeared in the early Devonian (400mya)

Distribution of lungfish

Three genera of lungfish are alive today; each is found on a single continent

This whopper is from Australia

Of the 5 major mass extinctions documented by the fossil record two are best studied – Permian and Cretaceous

The Permian mass extinction eliminated more than 97% of marine life.

Permian extinction was a period of global warming

• Mass Extinction 250 Million Years Ago Sparked Dramatic Shift To Complex Marine Ecosystems

• the event wiped out an estimated 95% of marine species and 70% of land species

• changed the basic ecology of the world's oceans• Probably took place in less than 5 million years• Most extreme period of volcanism in last half billion

years in area that is now Siberia

Complex marine ecosystems millions of years after the Permian extinction displaced the simpler ecosystems that had existed previously.

Cretaceous Mass Extinction (65 mya)

• Extinguish more than half of marine species

• Eliminated most dinosaurs

• Early primates survived• Cloud from possible

meteor would have blocked sunlight and disturbed climate

Creatures before and after the Cretaceous mass extinction (65 mya)

Ecological Consequences of Mass Extinctions

• Reduces a complex ecological community to a much reduced one; takes at least 5 - 10 million years to rediversify, sometimes 100 million

• Lineages (species) that disappear do not reappear• After the Permian and Cretaceous mass extinctions,

percentage of marine predators increased substantially

• Mass extinctions can allow new groups of organisms to become dominant species

Diversity of life has increased as a consequence of adaptive radiations

Results in new species whose adaptations allow them to fill different ecological niches

Adaptive Radiation of

mammals

Dwarf Cloud Rat

Millions of years ago

Monotremes(5 species)

250 150 100200 50

ANCESTRALCYNODONT

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Marsupials(324 species)

Eutherians(placentalmammals;5,010 species)

Ancestralmammal