When did the first fish live on Earth? How do scientists realize this?

The oldest fossils of fish-like animals date from between 518 and 530 million years ago. Discovered in China and called Haikouichthys, these animals were about an inch long (2.5 cm) and had a head with seven to eight slits at its base that looked like gills. They also had a distinct spine surrounded by muscles.

But there are ways Haikouichthys did not resemble any modern fish. For example, they had no jaw. Instead, their mouth was a cone-shaped opening similar to those seen in modern hagfish and lampreys. They also appear to lack side fins.

Reconstruction of Haikouichthys ercaicunensis based on fossil evidence. Talifero/Wikimedia Commons, CC BY-SA

Even though scientists like me weren’t around to see for ourselves what was happening on Earth so long ago, we use geological clues to determine which animals lived when. Here’s how we sort through very old timelines and even put dates on fossils like Haikouichthys.

Measuring in millions

To find out how long ago fish first appeared on Earth, you need a way to measure really, really long time intervals. Clocks measure short intervals, such as seconds, minutes, and hours. Calendars measure longer intervals, such as days, months, and years. What can you use to measure millions of years?

Radiometric dating is the method used by scientists 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.

You may know that atoms are the building blocks of molecules, which make up everything around you – grass, cement, and even air. While most atoms are very stable, some, called radioactive atoms, are unstable. Over long periods of time, they spontaneously break down into more stable atoms.

Uranium is one of these radioactive atoms. It breaks down very slowly into lead. Uranium and lead atoms can be found naturally in rocks and minerals in very, very small amounts.

Nuclear physicists have calculated that it would take 700 million years for a pound of uranium 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 the age of rocks and fossils fairly accurately.

Ernest Rutherford at McGill University, 1905. Unknown, published in 1939 in ‘Rutherford: being the life and letters of the Rt. Hon. Lord Rutherford’/Wikimedia Commons, CC BY

The idea of ​​radiometric dating first came to a New Zealand scientist named Ernest Rutherford in 1904. His idea was to measure the number of uranium atoms and lead atoms in a rock and compare them. He predicted that an older rock would contain more lead and less uranium than a younger rock.

Unstable atoms turn into stable atoms over time at a steady and predictable rate. NOAA

American scientist Bertram Boltwood put Rutherford’s idea to the test by measuring the amount of uranium and lead in different rocks collected from around the world.

Once a rock is formed, no new elements are added to it. So scientists can calculate how much uranium the rock started out with by adding what’s left to the amount of lead that’s there now, through the process of radioactive decay. Then, because they know exactly how long it takes uranium to break down into lead, they can determine the age of the rock. Boltwood proved Rutherford’s idea worked, establishing the field of radiometric dating in 1907.

The making of Haikouichthys fossil

Fossils are rocks. Thus, scientists can use radiometric dating to estimate how long the organisms that left the fossil footprint lived on Earth.

Animals leave fossil footprints only under special circumstances. So that the Haikouichthys to leave fossils, their corpses would have had to sink to the bottom of the water and be covered in sediment before the microorganisms could decompose them. Then, the minerals contained in the sediments would have seeped into the Haikouichthys for their remains to fossilize.

An almost complete specimen of Haikouichthys with the eye and zigzag-shaped muscle fibers called myomeres visible. It is one of many Haikouichthys fossils found in China. Dr and Prof. Degan Shu, Shannxi Key Laboratory of Early Life and Environment Department of Geology, Northwest University

Radiometric dating of Haikouichthys fossils suggest that these animals were swimming in Earth’s waters between 518 and 530 million years ago – and possibly longer.

The age of the earth in a 24 hour day

Scientists, using radiometric dating, estimate the Earth itself to be 4.5 billion years old. For a long time on Earth there was no life at all. Then, microorganisms like bacteria appeared. It was only relatively recently that plants and animals began to live on Earth.

In fact, if you consider the age of the Earth so far as a 24-hour day, it turns out Haikouichthys lived 2 hours and 45 minutes before the end of the day. Human-like animals appeared on Earth even more recently – around 5-7 million years ago – just minutes before the end of the hypothetical day.

If the Haikouichthys was the first fish or not remains controversial. There are very few other fish-like fossils from the same period. But paleontologists keep digging. Who knows, maybe in a few years they will discover an even older fish animal that will dethrone Haikouichthys as the oldest fish-like creature.

Isaac Skromne, Assistant Professor of Biology, University of Richmond

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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