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Wednesday, August 17, 2022

When did the first fish live on Earth – and how do scientists find out time?

Curious Kids is a series for kids of all ages. If you have a question you’d like an expert to answer, send it to curiousKids@theconversation.com.


How do you find out how long ago a fish was made? It was millions of years ago. – Josh, age 11, Ephrata, Pennsylvania


The oldest fossils of fish-like animals date back to 518 million to 530 million years ago. Invented in China and called haicauthis, these animals were about an inch long (2.5 cm) and had heads with seven to eight slits at the base that looked like gills. He also had a distinct spine surrounded by muscles.

but there are ways haicauthis Did not resemble any modern fish. For example, they did not have jaws. Instead, their mouth was a cone-like opening as seen in modern hagfish and lampreys. Looks like they didn’t have side fins.

Even if scientists like me weren’t around to see for ourselves what was happening on Earth not so long ago, we use geological clues to find out when animals lived. Here’s how we sort out very ancient timelines and even put dates on fossils like haicauthis,

measure in millions

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

Radiometric dating is the method 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 from which they are made.

You may know that atoms are the building blocks of molecules, which make up everything around you – grass, cement, 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 into lead very slowly. Both uranium and lead atoms can be found naturally in very small amounts in rocks and minerals.

Nuclear physicists have calculated that it would take 700 million years to break down one pound of uranium into half a pound of lead. This rate of decay occurs at such a predictable rate that scientists can use it to accurately calculate how old rocks and fossils are.

Black And White Photo Of A Man In Vintage Style Dress Sitting In Front Of An Elaborate Contraption.
Ernest Rutherford at McGill University, 1905.
Unknown, published in 1939 in Rutherford: Being the Life and Letters of the RT. Honourable. Lord Rutherford’/Wikimedia Commons, CC BY

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

A Graph Showing How The Proportion Of Unstable Atoms In A Substance Decreases While The Proportion Of Stable Atoms Increases With Time.
Unstable atoms turn into stable atoms with a constant and predictable speed over time.
NOAA

American scientist Bertram Boltwood tested Rutherford’s idea by measuring the amount of uranium and lead in various 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 introduced by adding up what is now left, thanks to the radioactive decay process. Then, because they know how long it takes for uranium to break down into lead, they can determine the age of the rock. Boltwood proved that Rutherford’s idea worked, establishing the field of radiometric dating in 1907.

the construction of haicauthis fossil

Fossils are rocks. So scientists can use radiometric dating to estimate how long ago the organisms that left the fossil imprint lived on Earth.

Animals leave traces of fossils only under special circumstances. for haicauthis To release fossils, their carcasses had to sink to the bottom of the water and covered with sediment before the microorganisms could decompose. Then, the minerals in the sediment will seep in. haicauthis so that their remains can become fossils.

A Close-Up Photo Of A Hycoichthys Fossil Labeled 'Eye' And 'V-Shaped Myomere'.
almost complete sample of haicauthis Eye and with crooked muscle fibers called myomeres. it is one of many haicauthis Fossils discovered in China.
Dr. and Prof. Daegan Shu, Shanxi’s Laboratory of Early Life and Environment Department of Geology, Northwest University

radiometric dating haicauthis Fossils show that these animals were swimming in Earth’s waters between 518 million and 530 million years ago – and possibly longer.

Earth’s age as a 24-hour day

Using radiometric dating, scientists estimate that the Earth itself is 4.5 billion years old. For a long time there was no life on Earth. Then microorganisms such as bacteria appeared. It is 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 haicauthis Lived 2 hours 45 minutes before the end of the day. Human-like animals appeared on Earth even more recently – about 5 million to 7 million years ago – just minutes before the end of the hypothetical day.

whether haicauthis Whether or not the first fish was there remains controversial. There are very few other fish-like fossils from the same time period. But paleontologists keep digging. Who knows, maybe in a few years they’ll find an even older fish-like animal that’ll be dethroned haicauthis As the oldest fish-like creature.


Hello curious kids! Do you have a question that you would like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and city where you live.

And since Curiosity has no age limit – adults, let us know what you’re thinking, too. We won’t be able to answer every question, but we will do our best.

World Nation News Desk
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