1. Determining Earth’s Age
Scientists have used radiometric dating to determine the age of Earth.
Meteorites do not break down like rocks on Earth and are believed to have been on Earth from the beginning.
Meteorites provide an unspoiled sample for radiometric dating.
Scientists measure the Earth’s age at about 4.5 Billion years.

2. The Geologic Time Scale
The Geologic Time Scale is a representation of the history of Earth.
It organizes Earth’s history by major changes or events that have occurred, using evidence from the fossil and geologic records.
The time scale is divided into units:
1. Eras – last tens to hundreds of millions of years and consist of two or more periods.
2. Periods – the most commonly used units of time on the geologic time scale, lasting tens of millions of years. Each period is associated with a particular type of rock.
3. Epochs – the smallest units and last several million years.

5. Earth was very Different Billions of Years ago
12.3: Origin of Life
Words: Nebula, Ribozyme
Earth was very Different Billions of Years ago
Most scientists agree on two points when it comes to the origin of the Earth:
1. Earth is billions of years old.
2. The conditions of the early planet and its atmosphere were very different from those of today.

6. Earth was very Different Billions of Years ago.
Today, the most widely accepted hypothesis of Earth’s origins suggests that the solar system was formed by a condensing Nebula, a cloud of gas and dust in space.
This idea is supported by research and suggests that the Earth is 4.6 Billion years old.
Early Earth was violent and very hot for the first 700 million years.
Hydrogen, carbon monoxide and nitrogen gas were in the atmosphere.
What was not present was OXYGEN.
As the planet cooled, water vapor condensed and fell as rain that collected as pools.
Once water was present, organic compounds could form.

7. Organic Molecule Hypothesis
There are two general hypotheses about how life-supporting molecules appeared on early Earth.
Miller-Urey Experiment
In 1953, Miller and Urey designed an experiment to test a hypothesis first proposed in the 1920’s.
Miller and Urey built a system to model early Earth.
They demonstrated that organic compounds could be made by passing an electrical current through a closed system of early gases.
These gases were Methane, ammonia, hydrogen and water vapor.

8. Meteorite Hypothesis
Analysis of a meteorite that fell in Australia in 1969 revealed that amino acids are present on meteors.
This evidence suggests that amino acids could have been present when Earth formed, or that these organic molecules may have arrived on Earth through meteorite or asteroid impacts.

9. Early Cell Structure Hypotheses
Iron-Sulfide Hypothesis
Martin and Russell noted that hot iron sulfide rising from below the ocean floor combines with the cooler ocean water to form chimney-like structures made of many compartments.
They proposed that 4 billion years ago, biological molecules combined in the compartments of these chimneys.

10. Early Cell Structure Hypotheses
Lipid Membrane Hypothesis
Several scientists have proposed that the evolution of lipid membranes was a crucial step for the origin of life.
Lipid molecules spontaneously form membrane-enclosed spheres, call liposomes.
In 1992, Harold Morowitz tested the idea that at some point liposomes were formed with a double, or bilayer, lipid membrane.
These liposomes could then form around a variety of organic molecules, such as amino acids, fatty acids, sugars and nucleotides.
The liposomes would act as membranes and would later give rise to the first true cells.

11. RNA as Early Genetic Material
A hypothesis that has become much supported recently states that RNA, rather than DNA, was the genetic material that stored information in living things on early Earth.
In the 1980’s it was discovered that RNA can catalyze reactions.
Ribozymes are RNA molecules that can catalyze specific chemical reactions.
RNA can copy itself, chop itself into pieces, and from these pieces make even more RNA.

12. 12.4: Early Single-Celled Organisms
Words to Know: Cyanobacteria, Endosymbiosis
MICROBES
Single-celled organisms changed Earth’s surface by depositing minerals.
They changed the atmosphere by giving off oxygen as a by-product of photosynthesis.

13. Before photosynthesis evolved, the first prokaryotes would have anaerobic, living without oxygen.
Scientists have found evidence that photosynthetic life evolved more than 3.5 billion years ago.
These fossils are of Cyanobacteria, which are bacteria that could carry out photosynthesis and release oxygen.
Some cyanobacteria live in colonies.
Higher oxygen levels in the atmosphere and ocean allowed the evolution of aerobic prokaryotes, which need oxygen to live.
Microbes

14. Eukaryotic Cells
The fossil record shows that eukaryotic organisms evolved 1.5 billion years ago.
Eukaryotes have a nucleus and other membrane bound organelles.
One hypothesis of eukaryotic evolution is the theory of endosymbiosis.
Endosymbiosis is a relationship in which one organism lives within the body of another, and both benefit from that relationship.
The Theory of Endosymbiosis suggests that early mitochondria and chloroplasts were once simple prokaryotic cells that were taken up by larger prokaryotes around 1.5 billion years ago.
Instead of being digested, some of the smaller prokaryotes may have survived inside of the larger ones.
If it the cell took in a prokaryote that acted like a mitochondria, the larger cell got the energy from ATP.
If the cell took in a prokaryote that acted like a chloroplast, the larger cell got nutrients through photosynthesis.
In exchange, the mitochondria and chloroplasts found a stable environment and nutrients.
Support for this theory is based on the fact that both chloroplasts and mitochondria have their own DNA and are the same size as prokaryotes.

15. Eukaryotic Cells

16. Evolution of Sexual Reproduction
The first prokaryotes and eukaryotes could only reproduce asexually.
Sexual reproduction may have resulted in an increase in the rate of evolution by natural selection.
Sexual reproduction creates more genetic variation.
Sexual reproduction may have been the first step in the evolution of multicellular life.

17. 12.5: Radiation of Multicellular Life.
Words to Know: Paleozoic, Cambrian Explosion, Mesozoic, and Cenozoic.
Life Moved onto Land
One hypothesis suggests that it was an advantage for early one-celled organisms to increase in size by becoming multicellular.
Cells that cooperated could compete more effectively for limited resources.
Multicellular organisms first appeared during the Paleozoic Era, which began 544 million years ago.

18. Life Moved onto Land
Members of every major animal group evolved within only a few million years.
The era ended 248 million years ago with a mass extinction.
More than 90% of marine animal species and 70% of land animal species of that time became extinct.
The earliest part of the Paleozoic Era, the Cambrian Period is often called the Cambrian explosion.
A huge diversity of animal species evolved.
The middle of the Paleozoic Era was a time of great diversity as life moved onto land.
The number and variety of plant groups greatly increased.
Four-legged vertebrates, such as amphibians, became common.

19. Reptiles Radiated During the Mesozoic Era
The Mesozoic Era began 248 million years ago and ended 65 million years ago.
Called the Age of Reptiles because the dinosaurs roamed Earth during this era.
The Mesozoic era also feature birds and flowering plants.
The oldest direct ancestor of mammals first appeared.
The Mesozoic era is divided into three periods: Triassic, Jurassic and Cretaceous.
Life took off slowly in the early Triassic.
The Jurassic was marked by the dinosaurs.
The peak in dinosaur diversity though is the Cretaceous.
This era ended with the most famous mass extinction when it is believed a meteorite struck the Earth and blocked the sun.

20. Mammals Radiated during the Cenozoic Era.
The Cenozoic Era began 65 million years ago and continues today.
It is divided into two periods: Tertiary and Quaternary.
During the Tertiary Period, placental mammals and monotremes evolved and diversified.
During the Tertiary period, birds, ray-finned fishes, and flowering plants also underwent dramatic radiations.
The earliest ancestors of the modern humans evolved near the end of the Tertiary.
However, homo sapiens anatomically modern humans, did not appear until about 100,000 years ago.

21. 12.6: Primate Evolution
Words to Know: Primate, Prosimian, Anthropoid, Hominid, Bipedal

22. Humans Share a Common Ancestor …
The common ancestor of all primates probably arose before the mass extinction that closed the Cretaceous period 65 million years ago.
Primates make up a category of mammals with flexible hands and feet, forward-looking eyes, and enlarged brains relative to their body size.
Primates also have arms that can rotate in a circle around their shoulder joint, and many primates have opposable thumbs.

23. Primate Evolution
The relationship of primate evolution forms a multi-branched tree.
Prosimians are the oldest living primate group, and most are small and active at night.
Ex: lemurs, lorises, and tarsiers.
Anthropoids, the humanlike primates, are further subdivided into the New World monkeys, and hominoids.
Many species have prehensile, or grasping, tails, that allows them to hang.
They have larger brains and can manipulate objects.
Hominids walk upright, have long lower limbs, thumbs that oppose, four other fingers and relatively large brains.
This group includes ALL the human lineage, both modern and extinct.
Walking Upright
Fossil discoveries have revealed that one trait had a huge impact on development – walking on 2 legs.
Bipedal is an adjective that describes two-legged or upright walking.
What is another common animal that is bipedal?

25. There are Many Fossils of Extinct Hominids
Most hominid species are classified into two group: the genus Australopithecus and the genus Homo
The earliest member of the genus Homo was Homo habilis. (“handy man”).
He lived million years ago in what are now Kenya and Tanzania.
Made stone tools and more closely resembled the modern human brain in shape.
H. neanderthalensis lived from 200,000 – 30,000 years ago, in Europe.
Some evidence suggests that he existed with Homo sapiens (modern day humans).

26. Modern Humans Arose about 100,000 Years Ago.
Fossil evidence reveals that the first Homo sapiens (modern humans) appeared in Ethiopia.
The Role of Culture
Human evolution is influenced by culture.
Objects such as tools demonstrate a steady trend of increasing sophistication and usefulness.
Evolution of the Human Brain
The human brain and skull size have both increased.
These traits evolved much faster in human than in other hominids.