Structure of the Earth obducted onto the continental crust and preserved as ophiolite sequences. Many rocks now making up Earth's crust formed less than 100 million (1) years ago; however, the oldest known mineral grains are about 4.4 billion (4.4) years old, indicating that Earth has had a solid crust for at least 4.4 billion years. Earth's mantle extends to a depth of 2,890 km, making it the thickest layer of Earth. The mantle is divided into upper and lower mantle. The upper and lower mantle are separated by the transition zone. The lowest part of the mantle next to the core-mantle boundary is
How the Snake Lost Its Legs many of the, "nuts and bolts of evolution, deeper than just natural selection," are on display in the book, revealing, "precisely what genetic/physiological mechanisms are being selected for." The molecular biologist Arnaud Martin observes that, "As children, we have all wondered about 'the How and the Why' of animal features, and if you are reading this it is in fact quite possible that a similar inquisitiveness still burns within you. The tone of "How the Snake Lost its Legs" finds its roots in the famous "Just So Stories" of Joseph Rudyard Kipling by tickling this curiosity with the formulaic "How