What do you know about life on Earth?

Portrait of scientist, Melanie Stiassny.

Hi, I’m Melanie Stiassny. I study life in the oceans, the largest, most diverse habitat on Earth. 

Did you know that life on Earth began in the oceans? First came true bacteria and probably archaea, followed by eukaryotes. Eukaryotes include protists, plants, animals, and fungi. Protists appeared first. Plants, animals, and fungi appeared much later. And it was only about 360 million years ago when some vertebrate animals began transitioning onto land!

A Timeline of Life on Earth

Oh no, the timeline is out of order! Can you reorder it from the oldest to the most recent? Just drag each box to the correct spot.

"Human ancestors emerged"
"Plants and fungi emerged"
"Some animals with backbones moved to land"
"Oldest evidence of life: true bacteria and possibly archaea"
"Animals emerged"
"Protists emerge"
"Some animals with backbones moved to land"
"Plants and fungi emerged"
"Oldest evidence of life: true bacteria and possibly archaea"
"Human ancestors emerged"
"Protists emerged"
"Animals emerged"

You got it! Explore the timeline below to see how life evolved on Earth. 

Timeline measured in billions of years with history of the life on earth plotted at various points
Timeline measured in billions of years with history of the life on earth plotted at various points

 Life on Earth Quiz

Life evolved over billions of years. And today, millions of unique species live on our planet. To get a glimpse of the biodiversity , I've picked a few examples of marine life to show you. Each belongs to one of the major groups that scientists use to classify life on Earth: true bacteria, archaea, protists, fungi, plants, and animals.

seashore with green seawater, and closeup showing microscopic algae

True bacteria are teeny tiny. They are so tiny that you can only see them with a microscope. These single-celled organisms lack a membrane-bounded nucleus  within their cells . True bacteria are found in all environments on Earth. Blue-green algae is a type of true bacteria that lives in the ocean.

Blue-green algae are so tiny that you can find up to 10 million of them in:


the entire ocean


in the Mediterranean Sea


in a tidepool


just one teaspoon of seawater

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You got it!

ANSWER: just one teaspoon of seawater

Besides being numerous, true bacteria are also very diverse. There are more than 10,000 known species. But there may be as many as 4 million different species!

Check out these other kinds of true bacteria that live in different habitats on our planet. 

7 circles containing various examples of true bacteria: some are round, some pill-shaped, some long, and some with tails.
Question 1 of 6
Two extreme microbes, both appearing round with a thin cell wall.

Like bacteria, archaea are also tiny single-celled organisms. Scientists once thought archaea, like the extreme microbes shown here, were rare and only lived in harsh environments.

Where were these extreme microbes found?


deep in the rainforest


at the peak of the Himalayas


near deep-sea volcanoes


on cave divers

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ANSWER: near deep-sea volcanoes

These extreme microbes live off chemicals spewed out of deep-sea volcanoes. They don’t need sunlight to live! Scientists now estimate archaea to be 50% of the species found in the open ocean. But the exact number of archaea species is unknown.

Check out these other kinds of archaea that live in different habitats.

5 circles showing various types of Archaea: some are long, some round, and some pill-shaped.
Question 2 of 6

Protists are a very diverse group of organisms that are found in all habitats on Earth. Most are single-celled organisms. Some are closer to fungi, some are closer to plants, and some are closer to animals.

Which of these is NOT a protist?

brightly-colored sea anemone, displaying numerous numerous soft tentacles.

sea anemone

microscopic image of a leaf-shaped diatom


A kelp forest with two kelp vines swaying in the foreground.

giant kelp

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You got it!

ANSWER: sea anemone

The sea anemone is actually an animal. Both the diatom and the giant kelp are marine algae, a kind of protist. Diatoms are microscopic creatures made of glass. They are used in many toothpastes to help make your teeth clean! And the giant kelp can grow to more 45 meters (150 feet) tall. Some are used to bond ingredients together in tasty treats like ice cream and jelly.

Check out these other kinds of protists that live in different habitats.

Five different protists.
Question 3 of 6

Many fungi are decomposers. They break down dead organisms and return nutrients to the environments. Scientists estimate that there are 1,500,000 species of fungi on Earth.

How many known species of fungi live in the oceans?









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ANSWER: 1,000

Scientists have found fungi in almost every habitat, but less than 0.1% live in the ocean. Marine fungi species tend to live in coastal environments. Most are microscopic in size, but some are bigger than small animals.

Check out these other kinds of fungi on land and in the ocean.

Five different fungi, including mushrooms, lichen and basket fungus.

Fungi collage left to right: Blackening waxcap fungus, © Russell Cooper/NaturePL/Science Source; Shelf fungus, Budak/CC BY-NC-ND 2.0; Marine lichen, © Nigel Cattlin/Science Source; Basket fungus, © Bernard Spragg; Violet coral fungus, Dan Molter/CC BY-SA 3.0

Question 4 of 6
Mangrove tree with many roots showing above water

Plants are living organisms that use sunlight to make their own food. There are over 300,000 described species so far. Almost all plants live on land, but a few, like mangroves, have adapted to living in the ocean.

What adaptations help mangroves survive in seawater?


they filter seawater to make their own supply of nearly salt-free water


their wide leaves collect and store rainwater


they absorb water vapor from the air


all of the above

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You got it!

ANSWER: they filter seawater to make their own supply of nearly salt-free water

Seawater contains enough salt to kill most flowering plants. Yet mangroves thrive on tropical shorelines. Many mangrove species filter salt out of seawater as it enters their roots. Some species excrete salt through glands in their leaves. These leaves become covered with dried salt crystals and taste salty if you lick them!

Check out these other kinds of plants that live on land and in water. 

Nine examples of plants, including; ferns, succulents, cacti, water lilies, grasses and more.
Question 5 of 6
Spiny lobster crawling on sandy sea floor

Animals range from microscopic tardigrades to giant blue whales . Most animals have nervous systems, which enable them to feel and touch. There are more than 1,200,000 described species. The spiny lobster is a type of animal that lives in the ocean.

How does the spiny lobster find its way home?


it retraces the footprints it had made


it uses the magnetic field of the planet


it remembers the pattern of the coral reef it lives in


it asks other ocean creatures for directions

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You got it!

ANSWER: it uses the magnetic field of the planet

These spiny invertebrates often leave their dens at night to forage over large areas of the ocean floor. But thanks to a unique navigation system, they have no trouble finding their way home, even in complete darkness! Like birds and sea turtles, spiny lobsters can navigate by reading small variations in Earth’s magnetic field.

Check out these other kinds of animals that live on land and in the ocean.

Various images of animals, including: a ladybug, elephant, frog, bird and fish.
Question 6 of 6
Nice try! Good job! Well done! A perfect score! You got out of 6 right on the first guess.
Image Credits:
background image, ©AMNH/Liz Vernon; Portrait of Melanie Stiassny, © Denis Finnin/AMNH; Timeline of life on earth, ©Liz Vernon/AMNH; Green seashore, Tom Archer/NASA; Microscopic blue-green algae, © Willem van Aken/CSIRO; Bacteria collage left to right: Lactobacillus, Dennis Kunkel Microscopy/Visuals Unlimited; Diphtheria, © Sanofi Pasteur; Staphylococcus, © NIAID; E. Coli, ARS/USDA; H. pylori, © Juergen Berger/Science Source; Streptococcus Viridans, © Dennis Kunkel Microscopy/Visuals Unlimited; Streptococcus A, © BSIP SA/Alamy; Extreme microbes, © Wolfgang Baumeister/Science Source; Archaea, © Steve Gschmeissner/Science Source; Intestinal archaea, Halobacterium, halophilic archaea, and methanotrophic archaea, © Dennis Kunkel Microscopy/Science Source; Protists collage left to right: Diatom alga, © Steve Gschmeissner/Science Source; Dinoflagellate, © Raul Gonzalez/Science Source; Radiolarian, Picturepest/CC BY 2.0; Phytoplankton, © Steve Gschmeissner/Science Source; Foraminifera, © Nano Creative/Science Source; Sea anemone, © Bernard Spragg; Diatom, © M.I. Walker/Science Source; Giant kelp, Public Domain/NOAA  Mangrove, © Werner Bollmann/AGE Fotostock; Plant collage left to right: Fern, Nick Ford/CC BY-NC-ND 2.0; Water lily, Paul VanDerWerf/CC BY 2.0; Gum tree buds, © Nigel Cattlin/Visuals Unlimited; Succulent, D. Hutchman/CC BY 2.0; Cacti, © Andreas Rose/Imagebroker/AGE Fotostock; Chapparal Yucca, Pluckytree/CC BY-NC-ND 2.0; Red maple tree, Stanley Zimny/CC BY-NC 2.0; Water grass, Brad Thompson/CC BY 2.0; Sundew, © Mark Freeth/CC BY 2.0; Spiny lobster, Adam/CC BY 2.0; Ladybug, Richard Giddins/CC BY 2.0; Coral reef, ©John Anderson/Alamy; Tubeworms, NOAA; Tree frog, Thomas Shahan/CC BY-NC-ND 2.0; Elephant, © iStockphoto; Agama lizard, Atamari/CC BY-SA 3.0; Meadowlark, Kevin Cole/CC BY 2.0; White marlin, © Doug Perrine/Alamy; Baby, © Kidstock/AGE Fotostock.