Over 150 million years ago, Redwood and Redwood-like trees covered most of the Northern Hemisphere. As the climate changed, redwoods thinned out and today there are only two species of redwoods in California: the coast redwood (Sequoia sempervirens), seen here in Muir Woods, and the Giant Sequoia (Sequoiadendron giganteum) found in Yosemite and Sequoia / Kings Canyon National Park. While the giant sequoia grows in greater bulk, the coast redwood grows taller, reaching recorded heights of up to 379 (tree located in Redwood National Park). This makes them the tallest living organisms in the world. The tallest coastal redwoods in Muir Woods are in the Bohemian and Cathedral Groves. The tallest in the park measures over 252 high with a maximum width of over 14. Redwoods can live to be 2000 years old. The average age of the coast redwoods in Muir Woods is between 600 and 800 years, with the oldest tree over coming in at over 1200 years. Redwoods grow best in moderate temperatures and need significant moisture to thrive. Part of the reason why redwoods thrive along the California coast is due to year round fog. This perennial fog condenses on the leaves and needles of the trees, drips to the ground and provides redwoods with annual moisture, even during the dry season. Redwoods are best known for their thick, spongy red bark. The bark itself can be from from 6 to 12 inches thick and insulates mature redwoods against fire damage. Redwoods get their color and name from the reddish-brown, bitter chemical tannin. Tannin makes the bark and wood resistant to fire and attack by fungi and insects.