4.56 Billion Years Ago

Solar Nebula Forms
A concentration of hydrogen gas and dust swirling around our young Sun.

Oldest Rocks on Earth
Continental crust (land!) had formed by this time. Samples from Australia have been dated to be as old as 4.4 billion years!

Impacts Events Dramatically Reduced in Number
Asteroids continue to strike Earth and the other planets, but not as frequently.

Earliest Evidence Indicating Life (?)
Scientists are debating the fossil evidence and chemical signatures suggested as indicators of life at this time.

Volcanic Activity on the Moon
While the Moon has been geologically inactive for many millions of years, the youngest volcanic flows are about 1 billion years old.

First Stable Continental Crust
In other words, things were calming down on Earth and stable land was forming.

Oxygen-"Rich" Atmosphere Begins to Develop
Photosynthetic organisms were pumping oxygen into the atmosphere.

Valles Marineris Begins Forming on Mars
The crust of Mars stretched and broke apart, creating a long, deep chasm — as long as the distance between California and New York!

Floods on Mars!
Water trapped beneath the martian surface as ice occasionally catastrophically melted and flooded across the surface, carving giant channels.

Significant Available O₂ in Atmosphere
Oxygen had been building up via photosynthesizing organisms, but chemical processes used most of it.

Formation of Ozone Shield
As oxygen became more abundant in the atmosphere, some of it formed an ozone shield, protecting Earth's surface from harmful ultraviolet rays.

First Slime Molds
This is here solely because it is kind of cool to think about!

Tallest Volcano in our Solar System Begins to Form
Olympus Mons, 22 kilometers (14 miles) high, began forming. Some lava flows are so fresh that they might be younger than 1 million years. If Olympus Mons were on Earth, it would almost fill the entire state of Arizona.

Supercontinent Rodinia Comes Together!
Okay, it took a long time for Rodinia to come together and pull apart again. Rodinia began forming about 1.9 billion years ago as the continents began to converge. By 600 million years ago, it had pulled apart.

Early Multicelled Critters
Ediacaran Fauna — funky-looking fossils found in Australia, China, Russia, Canada, and the United States. None had shells, so all we have are traces of these organisms with soft bodies. They look a bit like sea pens and jelly fish.

First Shells
Shells — and other hard parts — offer protection and support to organisms. They also preserve far better than soft tissue, giving us a rich fossil record. Starting about 540 million years ago, lots of different shelled organisms are found as fossils.

Earliest Chordates
Organisms with a notochord — a first step to a vertebra. A notochord is a stiff rod of dense tissue.

First Fish
The early fish did not have jaws or a vertebrae (they did have a notochord!). Many had plates covering all or part of their bodies. Some probably burrowed into the soft seafloor and dredged up their food.

First Land Plants
There MAY be fossil spores from plants as early as ~500 million years ago. Think of “mosses” when you think of the first land plants — these early conquerors were low to the ground and needed access to water to reproduce. Only bacteria, slime, etc., were on land before this!

First Insects
Not far behind the plants! Some fossil evidence for insects includes "bite" marks in plant fossils.

First Vascular Plants
Vascular plants have a plumbing systems for transporting water and nutrients. Early vascular plants required water to reproduce (think ferns and horsetails).

Forests!
Once they made it onto land and developed an efficient piping system and strong tissue to support high growth, plants quickly grew taller, creating forests with multiple levels of growth.

Whopping Big Lava Flows Cover the Surface of Venus
Unlike other terrestrial planets, much of the surface of Venus is not heavily cratered. It appears to have been covered and smoothed by lava flows between 300 and 500 million years ago.

First Amphibians
There may be tracks at 390 million years ago, indicating earlier amphibians. The first amphibians probably lived in shallow, heavily vegetated waters. They evolved their “legs” to maneuver around all the vegetation. So legs came first, then the amphibians headed onto land. Amphibians are cold-blooded terrestrial animals that need to return to the water to reproduce (indeed, many must stay in moist environments so that they don't dry out). These first amphibians are the ancestors to our frogs and salamanders.

First Gymnosperms (Seed-bearing Plants)
Gymnosperms don't need water for fertilization. They also invented the “pre-packaged” seed — self-contained with nutrients. This allowed the seed to stay dormant until conditions were right for sprouting. Seeds could be dispersed by organisms. The first gymnosperms were the ancestors of our pine trees, giant redwoods, and ginkos!

First Reptiles
Reptiles “invented” the self-contained egg, which includes all the things a developing embryo needs (nutrients, protection from drying out, shedding of waste). Unlike amphibians, which have to go back to water to lay their eggs, reptiles could live away from bodies of water. Reptiles are cold-blooded, terrestrial animals with skin that prevents them from drying out.

90% of all Marine Species Become Extinct
Marine organisms, amphibians, and plants are hit hard by the largest mass extinction event known in Earth history. The cause is debated and may be related to multiple events, including volcanism, global climate change, and ocean overturn. This event may have cleared the playing field of competitors and aided the rise of the dinosaurs. There's always a winner in an extinction . . .

Supercontinent Pangaea Assembled
All the continents came together (again!) to form one big land mass.

First Dinosaurs
Could be as late as 220 million years ago. These reptiles started out as small carnivorous critters walking around on two legs (bipedal).

First Mammals
+/– 30 million years; mammal features are challenging to identify in the fossil record. The first mammals were small, nocturnal critters. Early mammal fossils have some evidence of pits for whiskers and fur, suggesting another element of “mammal-ism” — that of being warm-blooded.

First Birds ( Protoavis texensis ) (???)
Protoavis may actually be an early dinosaur, and not the first bird. It shares many characteristics with the early dinosaurs, but may also have had feathers.

First (Undisputed) Birds ( Archaeopteryx )
Most scientists believe that Archaeopteryx , a relative of the dinosaurs, is the ancestor of today's birds (so think of T-Rex the next time you see a sparrow . . .). Archaeopteryx has some reptile characteristics (teeth, long bony tail, grasping claws on its wing) and some bird characteristics (feathers; bird-like hip; thin, hollow bones). It probably was a weak flyer.

First Angiosperms (Flowering Plants)
Lots of debate on the timing of this event! Gymnosperms are commonly pollinated by wind; angiosperms have flowers that attract insects and birds, increasing the opportunities for pollination. Pollinated seeds are encased in nutrients — giving us the fruit we enjoy!

Asteroid Smashes Into Earth; Extinction of Dinosaurs
And the winners are: The Mammals!

First Grasses
Grasslands became very widespread as our climate cooled in the last 45 million years.

Antarctic Ice Sheet Develops
Antarctica had separated from all other land masses. As global climate cooled, an ice sheet grew across the continent. The ice sheet is this old, but the actual ice is not ; the ice that makes up the ice sheet is constantly taken away by melting and as icebergs and renewed by falling snow.

Mediterranean Ocean . . . the Whole Thing . . . Dries Out
Messinian Salinity Crisis — a layer of salt more than 2 kilometers (1.2 miles) thick is deposited in the Mediterranean.

Giant "Armadillos" Roam Gulf Coast of North America
Really. These critters were about 1.5 meters (5 feet) across.

Uplift of the Colorado Plateau Initiated
The Grand Canyon is young geologically! As the Plateau rose, rivers cut down deeper and deeper, forming the canyon.

First Early Hominid
Ardipithicus ramidus

Northern Hemisphere Ice Sheets First Form
The most recent period of glaciation is initiated.

Humans Creating Calendars, Planting Crops, and Navigating Based on Stars, Moon, and Sun