Glossary

A trait or behavior that a living thing has evolved that helps it better survive in its environment.  Giraffes have the adaptation of a long neck to reach the leaves high up in trees.

A group of animals that can lay water-tight eggs.  All amniotes are also vertebrates, but only some vertebrates evolved this adaptation to become amniotes.

This adaptation allowed amniotes to live completely on land, out of the water.  Humans, turtles, and birds are all amniotes.  Humans are amniotes even though we don't lay eggs; we have an additional adaptation that allows us to give live birth instead.  Frogs and fish are not amniotes because they must lay their eggs in water.

A scientific diagram showing the evolutionary relationships between different living things.  If you start at the bottom of a cladogram and work your way to an animal at the top, you can trace its major adaptations over time. 

Animals are closely related if their branches meet up close to the top.  The closer to the bottom their branches meet up, the less closely related they are.

A group of animals that have a hole in their hip sockets that allow their legs to point straight downwards.  This adaptation helped dinosaurs run quickly.

All dinosaurs are also sauropsids, tetrapods, amniotes, and vertebrates.  A T. Rex and a penguin are two examples of dinosaurs.  A crocodile is not a dinosaur because its legs go out to the side rather than straight down.

A scientific theory that explains how new species come about through many small adaptations over time.  Animals adapt to their environment to better survive and pass on their genes.  Over millions of years, these adaptations build up, creating new species.  There are four main points to evolution:

1. Within each species, individual animals are all different from one another.  Scientists call this variation.

2. Animals produce far too many offspring for them all to survive.  (Think about what would happen if every chicken egg in the grocery store hatched!)

3. Animals must compete with one another for food, shelter, and a mate.  There's only so much to go around.

4. The animals whose variations best help them win the food, shelter, and mates are the ones who are able to survive and pass their genes on to the next generation.

Instead of laying an egg with a baby animal inside, some animals allow the baby to develop inside the mother.  When we say a mother is "pregnant," we mean that she has a developing baby in her womb.  Instead of an egg hatching, the mother gives live birth to the baby once it has developed. 

Humans and all mammals give live birth instead of laying eggs.  Birds, dinosaurs, lizards, and fish lay eggs instead of giving live birth. 

A group of animals that give live birth and nurse their young.  Technically, the adaptation that scientifically defines mammals is the use of three bones to form the inner ear.  These bones came from the jaw in more primitive animals.

Humans, rabbits, bats, and whales are all examples of mammals.  Birds, lizards, and fish are not mammals.  All mammals are also vertebrates, tetrapods, and amniotes. 

Mammals have the ability to produce milk to help feed their babies.  A mother nurses her babies by feeding them this milk, which has all of the nutrients a baby needs to survive.  The babies eat only milk for a while--sometimes for several months.  In the picture, a mother pig nurses her babies, who are all being fed with their mother's milk.  All mammals, even humans, nurse their young.

A group of tetrapods that evolved a pair of openings in the roof of their mouths.  Scientists still don't completely understand what these openings are for.  Sauropsids include all reptiles, dinosaurs, and birds.  Mammals and fish are not sauropsids.

A group of animals with four limbs.  Having limbs eventually helped animals leave the water to live on land, escaping predators in the water.  All tetrapods are also vertebrates.

Humans, wolves, bats, snakes, whales, and dinosaurs are all examples of tetrapods.  Even though some of these animals do not have four limbs, they evolved from ancestors that did have four limbs.  Fish and insects are not tetrapods.

A group of all animals with a skull and a backbone.  Humans, fish, and dinosaurs are all examples of vertebrates.  Jellyfish, insects, and worms are not vertebrates because they do not have a skull or a backbone.

The ability to stop water from entering or leaving.  Water-tight eggs keep in moisture so that the eggs do not dry out.  This allows the eggs to be laid on land instead of in water.  Water-tight skin (like ours) also keeps in most moisture, but an amphibian's skin is not water-tight, so it must spend time in a moist environment.