Old Earth Creationism (OEC) refers to several types of creationist belief with the common thread that they all agree with science about the age of the planet (4.6 billion years rather than 8,000-12,000) but believe that God initially created the universe. Old Earth Creationism is an umbrella term for several different types of creationism, including Gap creationism, Progressive creationism, Day-Age creationism, and theistic evolution. The rising popularity of Old Earth Creationism and the corresponding decline of Young Earth Creationism over the last few centuries can be attributed to scientific knowledge, including the discovery of fossils, radiocarbon dating, ice cores, geological evidence of Ice Ages, measurement of the speed of light, and many others. Today, many Christians and Jews believe in Old Earth Creationism.
For thousands of years after the writing of the Bible, most Christians, Jews, and Muslims believed in Young Earth Creationism, that the Earth was created approximately 8,000 years ago. This was measured by using the genealogies and ages chronicled in the Bible and estimating the duration of time between Adam and more contemporary figures whose birth and death dates are known. Young Earth Creationists believe that the world was created by God in six literal 24-hour days, during which the Earth and everything on it, including the ancestors of all currently-living plants and animals, were created out of nothing.
However, Young Earth Creationism suffered as contradicting scientific knowledge was progressively uncovered, much of which suggested that the Earth was billions of years old rather than about 8,000. This resulted in changes in the popular interpretation of the Christian creation myth as presented in the book of Genesis. One of the first variants to emerge was Gap creationism, which argues that there was a large gap between Genesis 1:1, the "first creation" ("In the beginning God created the heaven and the earth"), and Genesis 1:2-31 ("And God said, "Let there be light"; and there was light," etc.) This theory was popularized by Thomas Chalmers, an early 19th century divinity professor at the University of Edinburg and founder of the Free Church of Scotland.
Another popular form of Old Earth Creationism is Day-Age Creationism, which argues that the "days" in the Biblical creation story are metaphorical, and that these days may have actually lasted millions or billions of years. This reconciles Genesis with science somewhat. Another variant is Progressive creationism, which states that God created the Earth and life progressively, over the course of billions of years, and that when a new species emerges, it is due to direct intervention by God. Progressive creationists dismiss the notion of macroevolution or a universal common ancestor.
Another subvariety of Old Earth Creationism is theistic evolution. Theistic evolution is the most "modernist" variety of creationism, which essentially asserts that God created the world billions of years ago, and uses Darwinian evolution and natural selection as the mechanism whereby new species are created.