Chapter 26: Early Earth and The Origins of Life
This chapter is more about what we don’t know than what we do know. However, it is important to understand the process that has led us to the conclusions we have drawn to this point.
Life on Earth originated about 3.5 to 4 billion years ago. The Earth was formed 4.6 billion years ago.
- How do we know when life on Earth began?
- Stromatolites: mounds of striped (“banded”) sediment date back to 3.5 billion years ago. These stromatolites are strikingly similar to layered mats constructed by some modern bacteria (cyanobacteria). Therefore, we presume that they were made by bacteria.
- Fossils thought to be bacteria have also been found and date back to 3.4 bya.
- How did the first cells originate?
- Apparently, the first cells originated spontaneously from inorganic matter. This doesn’t happen today.
- What was the Earth like 4 bya?
- We are pretty sure that there was no oxygen and that the atmosphere was highly reduced. Gases predicted to be present at the time include: N2, H2O, methane (CH4), ammonia (NH3), CO, and CO2.
- In 1920’s Oparin and Haldane predicted that conditions of early Earth favored formation of organic compounds.
- In 1953, Miller and Urey supported this belief with an experiment. They combined reducing gases, water that evaporated and condensed (to simulated the water cycle), and an electric spark (simulating lightning or some other source of energy). The result: amino acids, sugars, lipids, nitrogen bases, and even ATP! (well some of these were from follow up experiments performed by others since then)
- What is the theory for how these molecules became full-fledged cells?
- Good question. We don’t really know but this is our best guess:
- small organic molecules formed in reducing atmosphere (above),
- monomers joined to form polymers,
- polymers aggregated to form droplets (termed protobionts) with some characteristics of life, and
- eventually a molecule of heredity originated.
- In order for steps two and three to occur, the organic molecules had to be concentrated in some way. Hence, soil and clay came into play! Clay has binding sites to which monomers can bind. This helped to concentrate monomers enough that polymers and eventually aggregates of polymers could be formed. Also, clay contains metal ions that can act as catalysts, speeding up the formation of polymers.
- Tell me more about protobionts.
- Protobionts (aggregates of organic polymers with some characteristics of life)
- Maintain separation between internal and external environment of “cell”
- Respond to osmotic forces.
- Some can store a membrane potential (electrical charge used to transmit nerve impulses)
- Some “morph”—pieces can bud off or fuse giving it the appearance of “swallowing” or “giving birth”
- What were the first living things like?
- Prokaryotic (bacteria)
- RNA was probably the genetic material
- Probably heterotrophs—they “ate” (maybe “absorbed” is better) organic monomers for food
- Anaerobic—oxygen was not present in atmosphere until it was put there by photosynthetic bacteria as a waste product. These did not evolve until later.
- How certain are scientists about all this stuff?
- Not very. Scientists debate many aspects of this all the time. A few of these debates are listed below:
- Maybe organic monomers formed elsewhere and were transported to Earth from outer space by meteorites.
- Was there a simpler hereditary system with the ability to copy itself that preceded the origin of RNA? Was RNA ever the hereditary material?
- Maybe life began near hydrothermal vents instead of in shallow pools at the surface.
- How do we categorize all the living things that are alive today?
- Five kingdoms: Animalia, Plantae, Fungi, Protista, and Monera
- Three domains: Eubacteria, Archebacteria, and eukarya
- Also, Protista is a sort of catch all category that isn’t very well defined.
- Protista is subdivided by “animal like” protozoans, “plant like” algae, and “fungus like” slime molds are related organisms.
Animalia / Plantae / Fungi / Protista / Monera
Eukaryote / Eukaryote / Eukaryote / Eukaryote / Prokaryote
Heterotrophic (ingesting) / Autotrophic / Heterotrophic (decomposers) / Both/either / Both/either
Multicellular / Multicellular / Multicellular except yeast / Unicellular except some algae / Unicellular
Specialized tissue / Specialized tissue / Specialized tissue / No specialized tissue / No specialized tissue
Mostly motile / Sessile / Sessile / Both / both