The History of Life on Earth

• The last universal common ancestor

  (LUCA) is common to all organisms that

  live, and have lived, on Earth since life

  began.
• The Earth came into being about 4.6 BYA (BYA).
• Primitive Earth atmosphere:

  ▪ Most likely consisted of:

  • Water vapor

  • Nitrogen

  • Carbon dioxide

  • Small amounts of hydrogen, methane, ammonia, hydrogen sulfide,

  and carbon monoxide

  ▪ Little free oxygen

  ▪ Originally too hot for liquid water to form.
• Monomers came from reactions in the atmosphere

  – Oparin/Haldane Hypothesis (early 1900s)

  » Suggested organic molecules could be formed in the

  presence of outside energy sources using atmospheric

  gases

  • Monomers came from reactions at hydrothermal vents
• Stanley Miller (1953)

  ▪ Conducted an experiment to test the Operin/Haldane

  hypothesis
• Iron-Sulfur World Hypothesis

  • Suggests organic molecules reacted with amino acids to form peptides in the presence of iron-nickel sulfides:
• Protein-First Hypothesis:

  • Assumes that protein enzymes arose first

  • DNA genes came afterwards
• RNA-First Hypothesis:

  • Suggests only RNA was needed to progress toward

  formation of the first cell or cells

  • DNA genes came afterwards
• Before the first true cell arose, there would have been a protocell or protobiont, the hypothesized precursor to the first true cells.
• Proteinoids are small polypeptides with catalytic properties.

  • When proteinoids are placed in water, they form microspheres, structures made of proteins with many properties of a cell
• Lipids placed into water form cell-sized double-layered bubbles called liposomes.
• Stages of the Origin of Life:

  1. Early Earth
  2. Inorganic chemicals
  3. Small organic molecules
  4. polymers
  5. protocells
  6. cell
  7. LUCA
• Fossils- Remains and traces of past life
• Paleontology is the study of the fossil record
• Most fossils are traces of organisms embedded in sediment.

  Sediment is produced by weathering and erosion of rocks

  ▪ Sediment becomes a recognizable stratum in the

  stratigraphic sequence

  ▪ Strata of the same age tend to contain the similar fossil assemblages (index fossils) that can be used

  for relative dating

  ▪ This helps geologists determine relative dates of embedded fossils despite upheavals (relative dating)
• Absolute dating assigns an actual date to a fossil

  • One absolute dating method relies on radiometric

  (radioactive) dating techniques
• The geologic timescale

  ▪ divides the history of the earth into

  • Eras

  • Periods

  • Epochs
• Life arose during the Precambrian
• The first cells came into existence in aquatic

  environments

  ▪ Prokaryotes

  ▪ Cyanobacteria fossils have been found in ancient stromatolites

  ▪ Photosynthetic cyanobacteria added oxygen to the atmosphere

  ▪ Aerobic bacteria proliferated in the oxygen-rich atmosphere
• Endosymbiotic Theory

  • Mitochondria were probably once free-living aerobic

  prokaryotes.

  • Chloroplasts were probably once free-living photosynthetic

  prokaryotes.

  • A nucleated cell probably engulfed these prokaryotes that

  became various organelles
• The Paleozoic Era

  ▪ Begins with the Cambrian Period

  ▪ Lasted over 300 million years

  ▪ Includes three major mass extinction events

  • Disappearance of a large number of taxa

  • Occurred within a relatively short time interval

  (compared to geological time scale)
• The ancestry of all modern animals can be traced to the

  Cambrian period based on molecular clock data
• Plants

  • Seedless vascular plants date back to the Silurian period

  • Later flourished in Carboniferous period

  ▪ Invertebrates

  • Arthropods were the first animals on land

  • Outer skeleton and jointed appendages pre-adapted them to

  live on land

  ▪ Vertebrates

  • Fishes first appeared in the Ordovician period

  • Amphibians first appeared in the Devonian period and

  diversified during the Carboniferous period

  ▪ A mass extinction occurred at the end of the Permian

  period
• Continental drift

  ▪ The positions of continents and oceans are

  not fixed
• Plate tectonics

  • Earth’s crust consists of slab-like plates

  • Tectonic plates float on a lower hot mantle layer

  • Movements of plates results in continental drift