The History of Life

Early Earth Was Inhospitable Very Hot. Radioactive or subject to Radiation. Volcanoes Atmosphere very little oxygen, lots of H 2 O, and CO 2 and N 2.

About 3.9 Billion Years Ago.. Earth cooled enough for the atmosphere to condense. Lots of rain, Lightning. Earths oceans formed. Then….

Scientists Propose The first Organisms appear in the oceans. Remember we were not there so there is no real evidence that this is when life started. But there is some interesting clues that suggest life began 3.5 billion years ago.

The oldest rock that have been found are 3.9 billion years old. Found in these rocks were fossils of bacteria. Hummmmm.

Fossils clues to the past The life found on the Earth today is a small fraction of all the species that have ever existed. About 99% are extinct. A fossil is evidence of organisms that lived long ago.

Paleontologists, scientists who study ancient life. Use fossils to determine the kinds of organisms that lived in the past. If a fossil has sharp teeth the animal might have been a carnivore or meat eater.

Bone can indicate how the animal moved, as well as the size. If they only find big bones in an area maybe there was a river that carried away the small ones.

Plant fossils can point to what the climate was like, if a fossil that resembles a present day plant that lives in a mild climate, the fossil may have lived in a mild climate too.

Fossils occur in sedimentary rock. For fossils to form, organism usually have to be buried in small particles of mud, sand, or clay soon after they die. These are compressed over time and harden into a type of rock called sedimentary rock.

Fossils are not found in metamorphic rock, because heat and pressure destroy the bones before they can become fossilized. Read the inside story page 381 in text.

The Age of a Fossil How to determine the age of a fossil. Relative dating. Radiometric dating.

Relative dating If you were to stack newspapers in a pile, as each days newspaper were added to the pile the stack would become taller.

If the stack were left undisturbed, the newspapers at the bottom would be older than the ones on top.

If sedimentary rock has not been disturbed, the layers at the top must be younger than the ones at the bottom. Any fossils found in the top layers must also be younger than the ones found in the bottom.

Using this method, scientist can determine the order of appearance and extinction of species that formed fossils in the layers. However relative dating cannot tell the exact age of the fossil.

Radiometric dating To find the specific age of a fossil scientists use a method called radiometric dating.

Using radioactive isotopes (unstable atoms), which decay over time giving off radiation. Scientists can use the decay rate to determine the age of the fossil.

As a radioactive isotope decays it changes into a non radioactive element. By comparing how much radioactive isotope is in a fossil to how much of the non radioactive element scientists can determine the approximate age of a fossil.

Scientists use the radioactive isotope Potassium-40. Potassium-40 has a half life of 1.3 billion years. That means if you have 1 pound of potassium-40, in 1.3 billion years you will have a ½ pound of potassium-40 and a ½ pound of a non radioactive element.

To determine the age of a fossil less than 50,000 years old scientists use the radioactive isotope carbon -14. Carbon -14 has a half life of 5730 years.

LAB DETERMINING A FOSSILS AGE. Page 394 in text.

Geologic Time Scale Divided into 4 eras. –Precambrian era –Paleozoic era –Mesozoic era –Cenozoic era

Each era is divided into periods. See figure 14.4 page The divisions in the geologic time scale are distinguished by the organisms that live during that time.

Not only did life appear during this time but the fossil record indicates that there were several occurrences of mass extinction. Mass extinction is when entire groups of organisms disappear from the fossil record.

Class Activity Mini Lab 14-2 After your tape is prepared read pages 384 to 387 and fill in which the different forms of life appeared and any extinctions of life.

Assignment SA 14.1 page 387 #1-6.

The Origin of Life Back in the “Good ole Days” people thought that life arose spontaneously. That is life came from non living things. They observed decaying meat produce maggots, mud produce fish, grain produce mice.

Today we laugh at such ideas, but back then if you put a sack of grain down in your empty cellar, the next day you would have mice in the cellar eating the grain.

Then in 1668, an Italian physician, Francesco Redi, disproved a common belief. That decaying meat produced maggots. See figure page 388 in text.

After Redi’s cool experiment most scientists concluded that large organisms did not spontaneously generate. But came from a preexisting parent.

However at the same time the microscope was becoming more and more popular and since the first microscopes were not very good scientists could not observe the reproductive behavior of the microscopic world….therefore…

Scientists concluded that small microscopic life DID reproduce spontaneously.