1. Chapter 14
History of Life

2. Biogenesis vs. Spontaneous Generation
Biogenesis: states that all living things come from other living things. It is our current understanding of the life process.
Spontaneous Generation: states that living things can arise from nonliving things. It is a belief held by many people during the 17th century
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3. Redi’s Experiment
Francesco Redi (1668): Experimentally determined that rotting meat does not spontaneously turn into maggots. Instead, maggots arise from eggs laid by other flies.

4. Spallanzani’s Experiment
Lazzaro Spallanzani (1767): Experimentally proved that microorganisms do not arise spontaneously from a “vital force” in air. Instead the arise from other microorganisms.

5. Pasteur’s Experiment
Louis Pasteur (1864): Experimentally proved (without doubt) that broth does not spontaneously turn into microorganisms by using an S-shaped flask.

6. Earth’s History
So if spontaneous generation does not happen on Earth today, then how did cell-based life arise in the first place?
Scientists believe the answer to this question lies in the scientific belief that Earth was a very different place billions of years ago.
We need to briefly trace Earth’s History to understand.

7. The Formation of Earth
5 billion years ago: universe was a swirling mass of gas and dust which, overtime, was pulled together by gravity to form the sun.
Planets formed from the remaining dust and debris as it circled the sun and collided repeatedly
Earth is estimated to be more than 4 billion years old

8. Radiometric Dating
Radiometric Dating: a technique used to establish the absolute age of an object by comparing the relative percentages of a radioactive isotope and a stable isotope.
Atomic Number = number of protons in the nucleus of an element. Atoms of the same element always have the same number of protons…but the number of neutrons can vary, creating isotopes.
Radioactive isotopes are unstable: their nuclei will release particles or radiant energy in order to become more stable. This is called radioactive decay.

9. Rates of decay have been determined for many radioactive isotopes and half-lives have been established.
Half-life = length of time it takes for one-half of any size sample of an isotope to decay to a stable form.
Carbon is a common element used in radiometric dating.

10. Carbon-14 Dating
Organic material can be dating by comparing the amount of carbon-14 (unstable) with the amount of carbon-12 (stable)
Living things constantly take in carbon (mostly C-12 and a small amount of C-14)
The ratio of C-12: C-14 in living things is known and fixed.
When a living thing dies, it stops taking in carbon. The decay of C-14, however, continues.
Thus the ratio will change: the amount of C-14 will decline with respect to the amount of C-12.
The half-life of C-14 is 5,730 years. We can use C-14 dating for objects less than 60,000 yrs old.
Isotopes with longer half-lives are used to date older rocks.

12. First Organic Compounds
Alexander Oparin: proposed the first idea on the origin of organic compounds. He hypothesized that the many gases present in Earth’s early atmosphere, under very high temperatures, might have formed simple organic compounds such as amino acids.
These would have collected in lakes and seas as Earth cooled, and over time, fueled by energy from lightening and UV radiation, could have formed larger molecules such as proteins.

13. Miller and Urey’s Test
Stanley Miller and Harold Urey (1953): Tested Oparin’s hypothesis with the “electric spark experiment”. They succeeded in producing amino acids from the gases believed to have been present in Earth’s early atmosphere.
Similar and more recent tests have produced amino acids, ATP and nucleotides from these same gases.

14. Organic Compounds from Space
In 1970, organic compounds were discovered in a recently fallen meteorite, suggesting that organic compounds exist in space and could have accumulated on early Earth with fallen meteorites.

15. From Molecules to Cell-like Structures
How did the organic molecules of early Earth give rise to cells?
Sydney Fox: discovered the spontaneous formation of cell-like structures known as microspheres and coacervates from solutions of organic molecules.
These structures are surrounded by membranes made of lipids and can take up certain substances from their environment. They can grow in size and form buds that break off.
This shows that some of the important properties of life could have arose without the direction of genes.
microspheres
coacervates

16. The First Life-Forms
The final transition from organic molecules to living cells is the origin of heredity.
A closer look at RNA has led scientists to discover that some types of RNA, called ribozymes, act as catalysts (similar to enzymes)
Some can catalyze their own replication and thus life could have begun with some form of self-replicating RNA

17. The First Cells
It is believed that the first true cells were probably anaerobic, heterotrophic prokaryotes
They fed on spontaneously formed organic molecules…and probably used them up quickly.
This would quickly favor autotrophs similar to today’s archaea

18. Archaea and Chemosynthesis
Archaea are unicellular prokaryotes that thrive under extremely harsh environmental conditions.
They get energy by chemosynthesis, using CO2 as a carbon source and oxidizing inorganic chemicals like sulfur.
The oldest fossils are those of archaea

19. Photosynthesis
3 billion years ago, living cells began to photosynthesize
The first photosynthetic organisms were similar to today’s cyanobacteria.
Cyanobacteria live in large colonies and form structures called stromatolites.
Fossil stromatolites are 3.5 billion years old

20. As the oxygen produced by photosynthesis reached the upper atmosphere, it formed ozone, which was essential for life to continue to evolve on Earth.

21. The First Eukaryotes
Endosymbiosis: Theory proposed by Lynn Margulis which states that eukaryotic cells developed from a mutually beneficial relationship between small, aerobic prokaryotes living inside larger, anaerobic prokaryotes

22. Endosymbiotic Theory
Mitochondria and Chloroplasts may have developed from this relationship.