About 4.5 billion years ago, our solar system (the sun and its planets) formed via the accretion of dust into spheres; Earth is therefore roughly 4.5 billion years old. Life began about 3.5 billion years ago, with the appearance of single-celled marine organisms; since then, evolution has continuously given rise to new forms of life. Increasingly large and complex types of multicellular life emerged, with one branch of evolution leading to fish.
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The term cold-blooded denotes that a creature's internal temperature is determined by its environment (in contrast to warm-blooded creatures, whose bodies maintain an internal temperature independent of their environment).
A tetrapod is a creature with four limbs; a limb is a jointed appendage that extends from an animal's body. 52 - "Hand tool: smelting", Encyclopedia Britannica.
An aquatic egg is laid in water, and thus lacks a hard shell. 49 - "Native Americans of Middle and South America", Encarta 2004.
Amphibians evolved into reptiles, which lay hard-shelled eggs. 50 - "History of technology", Encyclopedia Britannica. 51 - "Guns, Germs, and Steel: The Fates of Human Societies", Jared Diamond.
Throughout its history, the Earth has cycled between cold glacial periods (during which large masses of ice, known as glaciers, cover much of the world's land area) and warm interglacial periods (which lack widespread glaciers).
Humans have lived through many such cycles, which take place over thousands of years.
While glacial periods made survival more difficult, they also lowered the sea level, easing human colonization of the world.
We currently live in an interglacial period, which began ca. About 14 billion years ago, the Big Bang gave birth to a universe containing pockets of dust.
Consequently, reptiles were the first vertebrates that could live their entire lives on dry land.
The final two major vertebrate groups, birds and mammals, evolved from reptiles.
Some mammals came to abandon a ground-based life, evolving to swing among the trees.