Early Earth and the Origin of Life Chapter 4: Early Earth and the Origin of Life

Some major episodes in the history of life.

Clock analogy for some key events in evolutionary history http://deeptime.info/

1.  The earth was formed ~4.5 billion years ago 2.  It took ~500 million years for the crust to solidify. 3.  The oldest fossils of microorganisms 3.5 billion years old, embedded in rocks in western Australia                         3a.  Prokaryotes dominated from 3.5 to 2 billion years ago.      - During this time, the first divergence occurred: Bacteria and Archae                                    

Early and modern prokaryotes

Fossilized evidence of bacteria Stromatolites are fossilized bacterial mats. Many fossils of prokaryotes are found in layers that make up the prokaryotic mats. Today stromatolites are nearly extinct, living a precarious existence in only a few localities worldwide – the most famous location is Shark Bay in Western Australia. More recently, stromatolite colonies have been discovered in locations such as the Bahamas, the Indian Ocean, Yellowstone National Park, and Poza Azul Lake at Cuatro Cienegas, Mexico.  

Bacterial mats and stromatolites

4. Oxygen began accumulating in the atmosphere about 2 4.  Oxygen began accumulating in the atmosphere about 2.7 billion years ago.                         a.  Cyanobacteria are photosynthetic prokaryotes that are still present today à produced oxygen.  

Banded iron formations are evidence of the age of oxygenic photosynthesis – approximately 2 BYA in photo

5.  The oldest eukaryotic fossils are ~2 billion years old.                         a.  Symbiotic community of prokaryotes living within larger prokaryotes.          Mitochondria and chloroplasts   6.  The oldest fossils of multicellular organisms are ~1.2 billion years old.

Endosymbiosis theory (Lynn Margulis, 1970’s) Model for origin of eukaryotes (A) endomembrane system of eukaryotes may have evolved from specialized infoldings of plasma membrane of ancestral prokaryotes (B) chloroplasts are descendants of photosynthetic prokaryotes, probably cyanobacteria - proposed ancestors of mitochondria were endosymbiotic bacteria that were aerobic heterotrophs a. may have first gained entry into larger cells as undigested prey or internal parasites Be a good skeptic! Where’s the evidence?

Eukaryotic Origins A. Invagination of plasma membrane B. Endosymbiosis Symbiosis : An ecological relationship between organisms of 2 different species that live together in direct contact. How did this get started? prey or parasite

Evidence modern-day endosymbiotic relationships common among protists similarity between eubacteria & the chloroplasts & mitochondria of eukaryotes size inner membrane systems, enzymes, electron transport systems reproduction resembles binary fission circular DNA size: similar to prokaryotes inner membranes of chloroplasts & mitochondria may have been derived from the plasma membranes of endosymbiotic prokaryotes, having several of the same enzymes and electron transport systems that resemble those found on the plasma membranes of modern prokaryotes mitochondria and chloroplasts reproduce by splitting process reminiscent of binary fission in bacteria Based on molecular tools comparing genetic composition, nucleic acid sequences and percentage composition, DNA in organelles separate from genomic DNA & its composition more like that of prokaryotes than eukaroytes.

Note the presence of two types of cells (photosynthetic and hold fast) – evidence of specialization of cells functions, that are important for development of multicellular organisms

7.  The oldest animal fossils are ~700 million years old. a.  Animal diversity exploded ~540 million years ago.  

Fossilized animal embryos from Chinese sediments 570 million years ago.

           8.  Plants, fungi, and animals began colonizing land ~500 million years ago.                         a.  First plants transformed the landscape… b.  Then animals were able to take advantage of new niches                                      Mammals evolved 50 to 60 million years ago.

The Cambrian radiation of animals

B.  The origin of life 1.  First cells may have originated by chemical evolution involving 4 steps:                     a.  Abiotic (Non-biological) synthesis of small organic molecules (monomers)  C + H = organic molecule   b.  Monomers joined together to form polymers (proteins, nucleic acids) c.  Origin of self-replicating molecules (inheritance of traits)  proteins and polynucleic acids d.  Packaging of these organic molecules into protobionts.  Aggregates of abiotically produced molecules that maintain an internal chemical environment and exhibit some of the properties associated with life (i.e. metabolism, excitability).

2.  Evidence that supports the four-stage hypothesis for the origin of life                         a.  Oparin and Haldane in the 1920s  Abiotic synthesis of organic molecules is testable in the laboratory   Hypothesis:  Conditions on primitive earth favored chemical reactions that synthesized organic compounds from inorganic precursors. These conditions were different from what is now present and include: - Reducing environment (no oxygen, but instead H2O, CH4, NH4, and H2) = lots of free electrons that could be used to reduce carbon and produce organic molecules. - Energy from lots of lightning, UV radiation (no O2 to block UV rays from the sun) and volcanic activity (heat).

b.  Miller and Urey in 1953                                     i. Tested the Oparin-Haldane hypothesis by creating conditions in which there was an   - Atmosphere above warmed sea water that contained H2O, H2, CH4, and NH3 and - Electrodes that simulated lightning. - From this setup, they obtained organic compounds such as amino acids that were collected in cooled water.

The Miller-Urey experiment

The experiment - organic molecules could be created out of inorganic molecules. So…….why don’t we see this happening in today’s world? Any organic molecules that are now formed would be used up by living organisms. If microorganisms were created from these organic molecules in the early earth’s water bodies, this would have been an example of spontaneous creation! For much of history, man believed that living organisms could be created spontaneously from non-living material (e.g. flies from dead meat, geese from barnacles, etc.) This idea was refuted by Louis Pasteur in the 1860’s.

3.  RNA was probably the first hereditary material a.  Today, genetic information is usually stored as DNA, but some organisms such as viruses use RNA to store info.  

Short polymers of ribonucleotides can be synthesized abiotically in the laboratory. If these polymers are added to a solution of ribonucleotide monomers, sequences up to 10 based long are copied from the template according to the base-pairing rules. If zinc is added, the copied sequences may reach 40 nucleotides with less than 1% error. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 26.11

RNA catalysts, called ribozymes, remove introns from RNA. In the 1980’s Thomas Cech discovered that RNA molecules are important catalysts in modern cells. RNA catalysts, called 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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Laboratory experiments have demonstrated that RNA sequences can evolve in abiotic conditions. RNA molecules have both a genotype (nucleotide sequence) and a phenotype (three dimensional shape) that interacts with surrounding molecules. Under particular conditions, some RNA sequences are more stable and replicate faster and with fewer errors than other sequences. Occasional copying errors create mutations and selection screens these mutations for the most stable or best at self-replication.

RNA-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 ribosomes. If 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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

4.  The precursors of early life are known as Protobionts. a.  Protobionts form spontaneously in lab experiments from mixtures of organic molecules. b.  They contain RNA that codes for metabolic proteins. These protobionts absorb food and the proteins catalyze it to make energy which can be used for growth and division to daughter cells. c.  Natural selection would favor protobionts that grow and replicate. When the organic molecules in the earth’s water bodies were gone, the protobionts would “evolve” to either obtain energy by photosynthesis or predation.  It would only take the creation and evolution of one (1) protobiont to give rise to the all the different organisms we see today.

6. Natural section could refine protobionts containing hereditary information Once primitive RNA genes and their polypeptide products were packaged within a membrane, the protobionts could have evolved as units. Molecular cooperation could be refined because favorable components were concentrated together, rather than spread throughout the surroundings. Fig. 26.13 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

This 4.5 billion-year-old rock, labeled meteorite ALH84001, is believed to have once been a part of Mars and to contain fossil evidence that primitive life may have existed on Mars more than 3.6 billion years ago. The rock is a portion of a meteorite that was dislodged from Mars by a huge impact about 16 million years ago and that fell to Earth in Antarctica 13,000 years ago. The meteorite was found in Allan Hills ice field, Antarctica, by an annual expedition of the National Science Foundation's Antarctic Meteorite Program in 1984. It is preserved at the Johnson Space Center's Meteorite Processing Laboratory in Houston.

c.  Louis Pasteur in the 1860s                                     i.  Tested whether microorganisms emerge by spontaneous generation or by reproduction of existing microorganisms.   - Microorganisms grew in open containers of sterilized broth.

C.  Major lineages of life 1.  At first, two kingdoms were recognized à Plants and Animals.   2.  In 1969, Robert Whittaker developed a five-kingdom system à Plants, Fungi, Animals, Protists, and Prokaryotes (Monera).

Whittaker’s five-kingdom system

Our changing view of biological diversity