Formation of the Earth:


Recall the cell theory: cells are produced by other cells.


How did life on Earth first begin?


Scientists have formed hypothesis to try to explain this event, however, this is based on small amounts of evidence since no one existed on earth during this time.



1. Geologic evidence shows that Earth, which is about 4.6 billion years old, was formed by pieces of cosmic debris that were probably attracted to one another over the course of 100 million years. 

o    There was a collision with this planet and another large planet and this produced enough heat to melt the entire globe.


o    Once Earth melted, its elements rearranged themselves according to density; the densest becoming the core; the least dense moved to the outside and cooled (about 4 billion years ago) forming the crust. 

o    The least dense elements (hydrogen and nitrogen) formed the atmosphere.


o    The Earth’s early atmosphere probably contained  carbon dioxide, water vapor, and nitrogen. No oxygen!! Life could not exist in these conditions


2.  The earliest sedimentary rocks deposited in water have been dated to 3.8 billion years ago.


o    Earth’s surface cooled enough that water could remain a liquid and collect. 


o    Thunderstorms drenched the planet, and oceans covered much of the surface.


o    Oceans were brown due to iron (Fe) content


3.   Scientist Stanley Miller and Harold Urey’s experiments suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on a primitive Earth.


o    They produced amino acids, monomers of proteins, by passing sparks through a mixture of hydrogen, methane, ammonia, and water. 

oIn 1995, Miller produced nucleotides found in RNA.

o      This suggests how simple compounds found on the early Earth could have combined to form the organic compounds needed for life.

4.  Formation of Microspheres: Large organic molecules can sometimes form tiny bubbles called proteinoid microspheres. 

o   Microspheres are not cells but they are able to store and release energy easily.

o   Microspheres are selectively permeable to water.


5. Evolution of RNA and DNA: Studies suggest that RNA may have evolved before DNA.

o    Sometimes RNA sequences can help DNA replicate.

o    RNA can reproduce and grow by themselves.


6. Development of Free Oxygen: There are microscopic fossils of single-celled prokaryotes dating back 3.5 billion years ago (bya) that resemble modern bacteria.

o    These microfossils must have lived without oxygen.


o    Today there are Archaebacteria that live in extremely harsh environments that do not need oxygen. Ex.  Some live in thick mud and digestive tracts of animals, some live in the hot springs in the oceans, and others in very salty environments in Utah’s Great Salt Lake.

o    Photosynthetic bacteria became common in the shallow seas of the Precambrian Era.

o    These bacteria started producing oxygen about 2.2 bya.

o    Once oxygen started accumulating in the atmosphere, the environment started changing.

o    This caused some bacteria to die and some lived and flourished.  Some even went extinct if they weren’t capable of adapting to the new environment.


7. Origin of Eukaryotic Cells:


The endosymbiotic theory: Eukaryotic cells arose from living communities formed by prokaryotic cells.


o    Mitochondria and chloroplasts contain circular DNA similar to bacterial DNA.

o    Mitochondria and chloroplasts have ribosomes whose size and structure resemble that of bacteria.

o    Like bacteria, mitochondria and chloroplasts have many of the features of free-living bacteria such as being able to grow and reproduce independently of the cell.


8.  Sexual Reproduction and Multi-cellular Organisms:

o    Once single celled eukaryotes arose, such as protists, they began to reproduce sexually.  This development allowed for diversity of species and increased the rate of evolution of new species tremendously.


o    Most prokaryotes reproduce asexually, cloning their own DNA to the next generation.  This type of reproduction restricts genetic variation to mutations in DNA.

o    Sexual reproduction, however, shuffles and reshuffles genes in each generation; therefore the offspring never resemble their parents exactly.

o    Favorable gene combinations greatly increase the chances of evolutionary change in a species due to natural selection.

o    A few hundred million years later, the development of multi-cellular organisms (fungi, plants, and animals) arose from single-celled organisms.

o    Diversity then exploded.