1 Million Years
Dominated by the Pleistocene Ice Ages, the past million years have seen the migration of hominids, mastering the use of fire and tracking now-extinct animals such as Mastodon and Woolly Mammoth.

The entire Pleistocene Epoch, from 1.8 Ma (Million Years Ago) to 10,000 years ago is characterized by climatic oscillations and cycles of glaciation and melting.

10 Million Years
Over the past 10 million years, hominids evolved in and around the Mediterranean Basin, which was cut off from the Atlantic Ocean. Around seven million years ago, the basin dried out and reflooded several times, leaving salt deposits of up to 6,500 feet. These deposits, formed during an event known as the Messinian Salinity Crisis, created changes in ocean circulation felt around the world.

Hominids, including Ramapithecus (10-4 Ma) and Australopithecus (4-1 Ma) such as the famous Lucy, evolved during this period.

100 Million Years
The dinosaurs shown here lived during the Late Cretaceous Period (70 million years ago) after the dinosaurs had achieved their greatest variety, only five million years before dinosaurs became extinct. The largest abrupt event of the last 100 Ma occurred when a meteor hit the region of Yucatan causing mass extinction about 65 million years ago, triggering the extinction of the dinosaurs and beginning the reign of mammals. This shift is known as the Cretaceous-Tertiary (KT) boundary.

The transient Paleocene-Eocene Thermal Maximum at 55 mya and the more robust Early Eocene climatic optimum around 52 mya were caused by elevated greenhouse gas levels. The warming of the planet such anomalies as palm trees in Alaska and crocodiles in the Arctic. Mammals, including the lemurs which are early ancestors of primates thrive, migrating to Europe and the Americas from Asia around this time (Beard, 2002).

Around 34 million years ago, the Antarctic ice sheet began to form, and some 20 million years back, major modern mountain ranges such as the the Cordilleras, the Andes, and the Himalayan range were formed, with mammals becoming dominant.

1 Billion Years

During the past billion years, the Earth has undergone major continental shifts, such as the formation a billion years ago of the supercontinent Rodinia and its subsequent breakup 750 million years ago .

It has also undergone protracted intervals of severe glaciation, including the Sturtian glaciation 770-720 million years ago and the Varanger/Marinoan glaciation 610-575 million years ago when continental ice sheets existed at low latitudes.

Archaea and cyanobacteria dating back to over 2.5 billion years ago eventually led to the development of more complex organisms. Fossils of the earliest known soft-bodied metazoans (early Ediacaran fauna) date back to about 590 million years ago, and during the Cambrian period an explosion of biologic life led to the appearance of many diverse marine invertebrates with exoskeletons. Early fish arrived around 410 million years ago.

The largest mass extinction in Earth's history possibly caused by an asteroid's impact with Earth occurred around 250 million years ago that particularly affected marine vertebrates.

10 Billion Years

Earth has made an estimated 4.55 billion revolutions around the sun. Using the Earth's annual unit of time, scientists estimate the universe is another 10 billion years older than Earth.

Some seven billion years ago, galaxies and star systems began to evolve, with White Dwarf stars serving as pressure cookers to create most of the rest of the elements that we find in the atomic chart.

Once Earth and the rest of our solar system were formed, liquid water quickly developed with the planet's climate stabilizing to prevent all the water from completely freezing or evaporating-- creating an environment from which biologic life began to develop. Although less intense than today, solar radiation bombarded Earth which initially didn't have protective atmosphere to shield early life forms from this radiation.

Photosynthesis provided the means to transform solar energy, water and atmospheric carbon dioxide into fuel that could be stored for later use, and in the process they changed the composition of the atmosphere by removing carbon dioxide and replacing it with free oxygen (O²). By two billion years ago, atmospheric oxygen and likely ozone had already reached substantial levels. As researchers such as Lynn Margulis (1992) have shown, biological organisms such as microbes have long influenced climate on Earth.

The Beginning to Time
While our primary measures of time-- years and days-- did not exist prior to Earth's birth some 4.55 billion orbits of the sun ago, we use the measure of a year to extrapolate to the time before Earth existed. According to many cosmologists, the scientists who study the origins of the universe, a "Big Bang" event that spontaneously created the universe occurred some 14 or 15 billion years ago. The only "thing" that existed as matter was the element hydrogen, which over hundreds of millions of years began to break down into helium and generate photons which release light.

Was there time prior to this cosmic happening? See Cosmology for Beginners for more

Also see Resources Beyond 100,000 Years.