When did the first fish live on Earth – and how do scientists figure out the timing?

pema href="https://theconversation.com/us/topics/curious-kids-us-74795"Curious Kids/a is a series for children of all ages. If you have a question you’d like an expert to answer, send it to a href="mailto:curiouskidsus@theconversation.com"curiouskidsus@theconversation.com/a./em/p hr blockquote pstrongHow do you figure out how long ago fish were created? Hundreds of millions of years is a long time ago. – Josh, age 11, Ephrata, Pennsylvania/strong/p /blockquote hr pThe a href="https://doi.org/10.1038/46965"oldest fossils of animals resembling a fish/a date back between 518 million and 530 million years ago. Discovered in China and called emHaikouichthys/em, these animals were about an inch long (2.5 cm) and had a a href="https://doi.org/10.1038/nature01264"head with seven to eight slits at its base that looked like gills/a. They also had a a href="https://doi.org/10.1038/nature01264"distinct spine surrounded by muscles/a. /p pBut there are ways emHaikouichthys/em did not resemble any modern fish. For example, a href="https://www.science.org/content/article/fossils-give-glimpse-old-mother-lamprey"they didn’t have a jaw/a. Instead, their mouth was a cone-like opening similar to the ones seen in a href="https://nhpbs.org/wild/Agnatha.asp"modern hagfish and lampreys/a. They also a href="https://doi.org/10.1038/nature01264"appear not to have had side fins/a./p pEven though a href="https://scholar.google.com/citations?hl=enamp;user=w4GYLBMAAAAJ"scientists like me/a weren’t around to see for ourselves what was happening on Earth so long ago, we use geologic clues to figure out what animals lived when. Here’s how we sort out very ancient timelines and even put dates on fossils like emHaikouichthys/em./p h2Measuring in the millions/h2 pTo figure out how long ago fish first appeared on Earth you need a way to measure really, really long time intervals. Clocks measure short intervals, like seconds, minutes and hours. Calendars measure longer intervals, like days, months and years. What can you use to measure millions of years?/p pa href="https://cosmosmagazine.com/earth/earth-sciences/what-is-radiometric-dating/"Radiometric dating/a is the method that scientists use to calculate the passage of time in millions of years. To determine the age of rocks and fossils, scientists measure the type of atoms they are made of. /p pYou might know that atoms are the building blocks of a href="https://theconversation.com/what-do-molecules-look-like-184892"molecules, which make up everything around you/a – grass, cement, even air. While most atoms are very stable, a href="https://kids.britannica.com/kids/article/radioactivity/399579"some, called radioactive atoms, are unstable/a. Over long periods of time, they spontaneously break down into more stable atoms. /p pUranium is one of these radioactive atoms. a href="https://kids.kiddle.co/Uranium"It breaks down very slowly into lead/a. Both uranium and lead atoms can be found a href="https://kids.kiddle.co/Pitchblende"naturally in rocks and minerals/a in very, very low amounts. /p pNuclear physicists have calculated that it would take a href="https://www.livescience.com/39773-facts-about-uranium.html"700 million years for one pound of uranium/a to break down into half a pound of lead. This rate of decay occurs at such a predictable rate that scientists can use it to calculate fairly accurately how old rocks and fossils are./p

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