CHAPTER 26 EARLY EARTH AND THE ORIGIN OF LIFE

1. CHAPTER 26 EARLY EARTH AND THE ORIGIN OF LIFE

4. The Earth formed about 4.5 billion years ago No clear fossils have been found in the oldest surviving Earth rocks, from 3.8 billion years ago. The oldest fossils that have been uncovered were embedded in rocks from western Australia that are 3.5 billion years ago. Life on Earth originated between 3.5 and 4.0 billion years ago

5. The earliest organisms were prokaryotes. Relatively early, prokaryotes diverged into two main evolutionary branches, the bacteria and the archaea. Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years ago

6. Photosynthesis probably evolved very early in prokaryotic history. Cyanobacteria, photosynthetic organisms that split water and produce O2 as a byproduct, evolved over 2.7 billion years ago. This early oxygen initially reacted with dissolved iron to form the precipitate iron oxide. This can be seen today in banded iron formations. Oxygen began accumulating in the atmosphere about 2.7 billion years ago

7. The first clear evidence of a eukaryote appeared about 2.1 billion years ago. This is the same time as the oxygen revolution that changed the Earth’s environment so dramatically. Chloroplasts and mitochondrion evolved Eukaryotic life began by 2.1 billion years ago

8. A great range of eukaryotic unicellular forms evolved into the diversity of present-day “protists.” Multicellulareukaryotes evolved by 1.2 billion years ago

9. Recent fossils finds from China have produced a diversity of algae and animals from 570 million years ago, including preserved embryos.

10. Most of the major groups of animals make their first fossil appearances during the relatively short span of the Cambrian period’s first 20 million years. Animal diversity exploded during the early Cambrian period

11. Most orders of modern mammals, including primates, appeared 50-60 million years ago. Humans diverged from other primates only 5 million years ago Plants, fungi, and animals colonized the land about 500 million years ago

12. Spontaneous generation- life arises from nonliving matter In 1862, Louis Pasteur conducted broth experiments that rejected the idea of spontaneous generation even for microbes. Biogenesis- all life today arises only by the reproduction of preexisting life. The Origin of Life

13. One hypothesis is that chemical and physical processes in Earth’s primordial environment eventually produced simple cells. (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”

14. 1920’s, A.I. Oparin and J.B.S. Haldane 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. The energy required could be provided by lightning and the intense UV radiation that penetrated the primitive atmosphere. 1.Abiotic synthesis of organic molecules

15. 1953, Stanley Miller and Harold Urey The atmosphere model consisted of H2O, H2, CH4, and NH3. It produced a variety of amino acids and other organic molecules.

16. Monomers should link to form polymers without enzymes and other cellular equipment. Researchers have produced polymers, including polypeptides, after dripping solutions of monomers onto hot sand, clay, or rock. 2. Laboratory simulations of early-Earth conditions have produced organic polymers

17. Many researchers have proposed that the first hereditary material was RNA, not DNA. RNA can also function as an enzyme Short polymers of ribonucleotides can be synthesized abiotically in the laboratory. 3. RNA may have been the first genetic material

18. 1980’s Thomas Cech- discovered that RNA molecules are important catalysts in modern cells. Ribozymes- RNA catalysts that remove introns from RNA. Ribozymes also help catalyze the synthesis of new RNA polymers.

19. Protobionts- maintain an internal chemical environment from their surroundings and may show some properties associated with life, metabolism, and excitability. Liposomes- droplets of abiotically produced organic compounds that form from lipids. Forms a molecular bilayerlike the lipid bilayer of a membrane. 4A. Protobionts can form by self-assembly

21. If enzymes are added, they are incorporated into the droplets. The protobionts are then able to absorbsubstrates fromtheir surroundingsand release theproducts of thereactions catalyzedby the enzymes.

22. Once primitive RNA genes and their polypeptide products were packaged within a membrane, the protobionts could have evolved as units. The most successful protobionts would grow and split, distributing copies of their genes to offspring. Later, DNA would replace RNA for storing genetic information. 4B. Natural section could refine protobionts containing hereditary information

23. Laboratory simulations cannot prove that these kinds of chemical processes actually created life on the primitive Earth. They describe steps that could have happened. Among the debates is whether organic monomers on early Earth were synthesized in deep-sea vents or reached Earth on comets and meteorites. The leap from an aggregate of molecules that reproduces to even the simplest prokaryotic cell is immense, and change must have occurred in many smaller evolutionary steps. Debates about the origin of life abounds

24. 1969, R.H Whittaker, A five-kingdom system: Monera, Protista, Plantae, Fungi, and Animalia. The five-kingdom system reflected increased knowledge of life’s diversity

25. Three-domain system: Bacteria, Archaea, and Eukarya, as superkingdoms. Arranging the diversity of life into the highest taxa is a work in progress