Download Chapter 6 – The History of Life on Earth

Chapter 6 – The History of Life on Earth
6.1 - The Earth has been home to living things for about 3.8 billion years.
1. Fossils provide evidence of earlier life.
- Fossils are preserved plants and animals as well as traces of plants and animals.
For example: footprints, feces, gnaw marks, and root holes.
- Fossils are not necessarily difficult to find; they are difficult to form.
- In order for a fossil to form, there must be
1. rapid burial
2. little or no oxygen
3. no disturbances
- Scientists think that only a tiny fraction of the countless organisms that lived on
Earth have been preserved as fossils. This is why we have so many gaps in the
fossil record.
- The oldest fossils that scientists have are prokaryotes (a cell without a nucleus)
that are more than 3 billion years old.
- There are 2 ways to determine the age of fossils:
Relative Dating vs. Absolute Dating
gives an approximate age
of the fossil according to
its position in the earth’s
rock
gives an age in years
using radioactive elements
found in the rock
surrounding the fossil
2. More complex organisms developed over time.
- Unicellular organisms appeared on Earth about 3.8 billion years ago.
- Multicellular organisms appeared on Earth about 1.2 billion years ago.
- These first organisms lived in oceans for about 3 billion years and finally moved
to land about 500 million years.
- The first land-dwelling organisms were simple plants and fungi.
3. Earth’s history includes mass extinctions.
- Mass extinctions occur when large numbers of species die or become extinct in a
short period of time.
- Permian Extinction – 90% of ocean-dwelling species became extinct.
- Cretaceous Extinction – dinosaurs became extinct
6.2 - Species change over time.
1. Scientists explore the concept of evolution.
- In the 1800’s, Lamarck proposed that organisms acquired traits during their
lifetimes, then passed these traits to their offspring but could not find evidence to
support this idea.
- About 50 years after Lamarck, Darwin made observations during a 5 year voyage
on the Beagle that formed the basis of his theory of evolution.
2. Natural Selection explains how living things evolve.
- Darwin based his theory of natural selection on his observations and his
knowledge of artificial selection (selective breeding).
- The 4 main principles in his theory include:
1. overproduction
(during reproduction organisms usually makes more offspring than the environment can support)
2. variation (within a species there are natural differences)
3. adaption (sometimes mutations can occur that make an individual better able to survive)
4. selection (individuals with certain adaptations are more likely to survive and reproduce)
- Traits that appear in populations due to genetic variation may help a population
survive.
- After many generations, the variation leads to adaptation, an inherited trait that
gives an organism an advantage in its environment.
- This advantage over other members of the population allows the population to
evolve through natural selection.
3. New species develop from earlier species.
- Speciation is the evolution of new species from existing species, occurs when a
population of species becomes isolated from other members of their species.
- Isolation can be geographic, reproductive, or behavioral.
6.3 - Many types of evidence support evolution.
1. Observations provide evidence for theories.
- A scientific theory is a widely accepted statement based on scientific evidence
that helps explain a group of facts.
- The theory of evolution has been built on fossil evidence, biological evidence, and
genetic evidence.
2. Fossil evidence supports evolution.
- Fossil evidence shows that 2 species with a common ancestor can develop
differently.
3. Biological evidence supports evolution.
- Vestigial Organs are physical structures that are no longer useful to a living
organism, but may have existed as fully developed similar structures useful to a
common ancestor.
- Vestigial Structures are remnants of once-useful structures (ex. modern whales
do not have hind limbs, there are remnants of hind-limb bones inside their bodies)
- Homologous Structures are structures that have similar origins and exhibit
similar anatomical patterns (ex. bird wings, human arms, whale flippers, and deer
forelimbs are all similar in skeletal structure)
- Embryonic Development similarities also indicate common ancestry.
4. Genetic evidence supports evolution.
- Similar organisms have very similar genes.
- Organisms with similar patterns of DNA share a common ancestor.