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Scientists have discovered the female sex hormone progesterone in a walnut tree, shaking up whats known about the different between plants and animals.

“The significance of the unequivocal identification of progesterone cannot be overstated,” write Guido F. Pauli and colleagues in the American Chemical Societys Journal of Natural Products. “While the biological role of progesterone has been extensively studied in mammals, the reason for its presence in plants is less apparent.”

Until now, scientists thought that only animals could make progesterone. A steroid hormone secreted by the ovaries, progesterone prepares the uterus for pregnancy and maintains pregnancy. A synthetic version, progestin, is used in birth control pills and other medications.

Scientists previously identified progesterone-like substances in plants and speculated that the hormone itself could exist in plants. But researchers had not found the actual hormone in plants until now. Pauli and colleagues used two powerful laboratory techniques, nuclear magnetic resonance and mass spectroscopy, to detect progesterone in leaves of the Common Walnut, or English Walnut, tree. They also identified five new progesterone-related steroids in a plant belonging to the buttercup family. Plants Make Own Painkillers Plants Recognize Rivals and Fight, Play Nice With Siblings Houseplants Make Air Healthier Original Story: This Trees a Lady!LiveScience.com chronicles the daily advances and innovations made in science and technology. We take on the misconceptions that often pop up around scientific discoveries and deliver short, provocative explanations with a certain wit and style. Check out our science videos, Trivia & Quizzes and Top 10s. Join our community to debate hot-button issues like stem cells, climate change and evolution. You can also sign up for free newsletters, register for RSS feeds and get cool gadgets at the LiveScience Store.

They speculate that the hormone, like other steroid hormones, might be an ancient bioregulator that evolved billions of years ago, before the appearance of modern plants and animals. The new discovery may change scientific understanding of the evolution and function of progesterone in living things.