1. Ch. 12 history of life on earth

2. Most scientists think that the earth was formed from the Big Bang about ~14 billion years ago
Radiometric dating has been used to calculate the age of the earth, using radioisotopes that give off radiation
A half-life is the amount of time it takes for a chemical to release half of its energy, or to decay
Scientists use half-lives to work backwards to calculate age of various objects like rocks and fossils

3. Primordial Soup Model
Hypothesis that the earth’s early ocean contained many different molecules with carbon
These organic molecules spontaneously joined in chemical reactions with energy supplied by solar radiation, volcanoes, and ocean vents
In 1953, Miller and Urey tested this hypothesis by placing gases they thought were present in early earth’s atmosphere in a glass container and provided an electrical shock
A few days later, simple compounds did come together from the carbon, but life was not formed
Today, scientists know that the gases used were not present in early earth’s atmosphere

4. First Cells/Life Bubble model Outer Space Model First Cells/Life
Hypothesis that gases from underwater vents and volcanoes rose to the ocean surface and formed molecules that led to life
Outer Space Model
Hypothesis that bacterial life came on asteroids on earth
First Cells/Life
Scientists propose that mRNA developed first and made proteins, and that DNA was made from RNA
The first cells were prokaryotic cells
Prokaryotes
Unicellular, no nucleus, no membrane-bound organelles
Consists of 2 groups: Eubacteria (bacteria that cause disease) and Archaebacteria (all other prokaryotes)

5. First Cells/Life Prokaryotes Eukaryotes
Scientists think that the first cell was a prokaryote called cyanobacteria, which does photosynthesis
Eukaryotes
Multicellular, has a nucleus, has membrane-bound organelles
Lynn Margulis theorized that the mitochondria and chloroplast organelles used to actually be prokaryote cells, and became engulfed by another cell forming a eukaryotic cells. This is called the endosymbiosis theory
Her proof was that those organelles are the same size as prokaryotes, those organelles have their own DNA and ribosomes, and those organelles divide like prokaryotes by binary fission during mitosis
Scientists think that the first eukaryotic multicellular organisms were called protests (paramecium)
Protests then gave rise to plants, fungi, and animals

6. First Cells/Life Eukaryotes
Multicellularity allows for cell diversity in life processes, which originated in the Cambrian period, million years ago
In 1909, the Cambrian “explosion” was discovered where almost all modern organisms were preserved in the Cambrian layer, and hardly any fossils are found before this time period in the fossil layer

7. Life to land
Scientists say that life started in the oceans and moved to land when the ozone layer formed in the atmosphere
Plants were the first organisms on land where they formed mutualistic relationships
Arthropods were the first animals on land
They have a hard outer shell, called an exoskeleton, a segmented body, and jointed limbs. Ex: lobsters, insects
After arthropods, vertebrates developed, which are animals with a backbone vs. invertebrate without one
Fish developed in the sea and amphibians developed from fish
To do this, fish had to develop legs, lungs Ex: frogs, salamanders
Reptiles evolved from amphibians and had to develop thicker skin, ability to gain heat from the sun, and laying eggs in the soil. Ex: Snake, lizard, turtle

8. Life to land Birds developed from reptiles, possibly from dinosaur
Mammals developed last and are found on land and in the ocean
Different populations of organisms are due Pangea