Dr. Universe: What caused the first mass extinction on Earth? - Aayush, 10, New Jersey
It's sad when living things go extinct. That means they're gone forever. I think about extinct unicorns all. the. time.
My friend Jodi Rosso told me that a mass extinction is when a huge number of species die out all at once. She teaches earth science at Washington State University.
"There are periods when a whole lot of animals and plants die off in a short time," Rosso said. "When we say a short time, we mean less than 2 million years. From a human perspective, that's forever. But in geologic time, that's short."
Groups of living things are always expanding or shrinking their numbers. Scientists think there are as many as 10 million species on Earth right now. Between 100 and 10,000 of them go extinct every year.
A mass extinction is when big percentage of all species die off.
The best-known mass extinction is probably the one that killed the non-avian dinosaurs. An asteroid crashed into the Earth 66 million years ago. It caused changes that wiped out about 75% of all living things.
But the first major mass extinction was way before that - about 443 million years ago. It's called the Late Ordovician mass extinction. Back then, most living things were in the ocean. About 85% of them died.
Scientists think it happened for two reasons. First, it was a time when the glaciers were getting bigger. As more water froze, there was less water in the ocean. Things living in shallow water had nowhere to go. Then, other changes reduced the amount of oxygen in the ocean.
"And little critters were like, 'Well, dang. Now my habitat is gone, and my oxygen is gone. I am not in a happy place,' " Rosso said. "So, it's usually not one thing. It's like one thing happens and then, maybe a couple million years later, another thing happens, and that combination really does a job on life."
But there were probably mass extinctions before that one. One way scientists figure this stuff out is by looking at the fossil record. Maybe there were lots of fossils and then - bam - there's a time with way fewer fossils. Or one kind of fossil goes away forever.
Fossils form when an organism gets buried quickly. Its hard parts - like bones and shells - turn into rock. But lots of living things don't have any hard parts. They're squishy.
Those living things probably won't show up in the fossil record anyway. So, it's a lot harder for scientists to know if soft-bodied creatures got wiped out.
That probably happened about 2.4 billion years ago after the Great Oxygenation Event.
Back when Earth formed, the air was full of carbon dioxide and methane. Then, some creatures called cyanobacteria began doing photosynthesis. They sucked in carbon dioxide and spit out oxygen. That oxygen added up until it became the second most common gas in the air. That's how oxygen-loving animals like us were able to evolve.
But there were probably lots of creatures that couldn't survive that change. Without hard bones or shells, it's hard to know how many died off.
Some mass extinctions are super obvious - like losing those big dinos. But some are mostly fos-silent in the geologic record.
Adults can help kids submit a question at askdruniverse.wsu.edu/ask.