What are we to make of “natural law”? It has become front and center in city halls, the media and the courts over same-sex marriage, civil union and homosexuality. We also hear it bandied about when inscriptions of the Ten Commandments are being removed from courtrooms and government grounds, and when teaching evolution in the science classroom is tossed in favor of creationism and “natural law.”

Are there such things as natural laws? Absolutely. Evolution by natural selection is as natural as laws get. It’s been governing life on Earth for more than 4 billion years. Continental drift and plate tectonics are natural laws that have shaped the landscape of the Earth, its mountains, oceans, continents and islands, over the same period.

Science has discovered hordes of natural laws: Newton’s laws of motion; Einstein’s laws of relativity; Boyle’s law of pressure and volume; Faraday’s laws of electromagnetism; Hubble’s law of distance and speed in the universe; Kepler’s laws of planetary movement. There’s at least one natural law for every letter of the alphabet. Check out the Laws List (http://www.alcyone.com/max/physics/laws/).

When people proclaim that natural law renders same-sex marriage unnatural, they’re not citing Darwin or Einstein. They’re citing Leviticus. In the newspaper and broadcast media, people who cite “natural law” imply they mean God’s law — what they believe God said, whom He said it to and where it is written. The source is scripture. Yes, Leviticus renders homosexuality an abomination. But is this the can of strictures we want to open in the name of natural law? Leviticus also prohibits any contact with a woman who is menstruating. Is that a natural law we should legislate? Exodus forbids working on the Sabbath, which is punishable by death. Is that, too, a natural law?

The larger issue does not require biblical scholarship. Leviticus, Exodus and the other three books of the Pentateuch prescribe 613 individual do’s and don’ts. Are they all natural laws by which we should still abide? If not, which are and who decides?

For example, as with homosexuality, Leviticus condemns the eating of seafood (“shellfish”) as an abomination. Leviticus also considers mixing of any kind a violation of natural law. It prohibits sowing a field with different crops, wearing clothes made of two different fabrics, or the breeding of cattle of different kinds. Forget organic farming, spandex, and all other artificially bred cattle. Leviticus instructs us to purchase our slaves from neighboring countries. Born in Canada, I’m fortunate Kansas was a Free State.

The point is not to poke fun at Leviticus. The point is that we cannot blithely pick and choose our sacred cows from the herd to justify a personal value. Otherwise, natural law merely becomes the biblical verse that fits one’s prejudice. Religion and scripture deserve better. Leviticus prescribes a code of laws for a people and society that lived in the Middle East 5,000 years ago.

None of these laws are “natural” in the sense of being everlasting and absolute. Some of them remain fundamentally wise and endure today, such as the prohibitions against murder, theft and incest. But many are anachronisms in modern society, made obsolete by knowledge and social change that have guided the evolution of our own body of law. After all, the first half of Leviticus deals with the “natural laws” governing animal sacrifice in worship.

Let’s leave natural law to nature. Natural law has a realm more spacious than human affairs and a rule more expansive than one species, Homo sapiens. Natural law is grand enough to explain the universe and its parts over time and space. And natural laws operate whether we are on Earth or not. Einstein’s laws began working when the first light emerged at the birth of the universe 13 billion years ago. Newton’s laws were working 65 million years ago when dinosaurs toppled over and shook the earth in an equal and opposite reaction. Darwinian evolution was working on millions of plants, animals and bacteria long before humans became part of the African landscape two million years ago.

The laws we codified since then tell a remarkable story of diverse peoples and populations — how their physical and social adaptations changed across time, cultures and continents. One of those adaptations was the ability to sense the universe around us and discover its natural laws. They are the common knowledge of ultimate origins and ultimate fate that unites us and by which we must now abide.

Leonard Krishtalka is director of Kansas University’s Natural History Museum & Biodiversity Research

Center.