Deuterostome

Some animals grow in a special way.

When they are very small, they make a first opening. In this group, that opening becomes the back end. Later, a mouth forms on the other side.

This group is very large. It includes fish and even humans.

It also includes sea stars and sea urchins.

Some of these animals live in the sea.

They have many different shapes. Some look like worms. Others look like stars.

Scientists found these animals in old rocks. These rocks are called fossils. They show us how animals lived long ago.

It is amazing to see how life grows!

Some animals grow in a very special way. This group is called deuterostomes.

When these animals are tiny embryos, they make a first opening. This opening is called a blastopore. In deuterostomes, the blastopore becomes the anus. The mouth forms later at a different spot. This is different from a group called protostomes. In those animals, the mouth forms first.

Deuterostomes are a big group of animals. One part is the chordates. This group includes fish and even humans.

Another part is the echinoderms. These animals live in the sea. They include sea stars and sea urchins.

A third part is the hemichordates. These animals often look like worms.

Scientists study old fossils to learn about them. They found fossils of early fish and early sea stars. These fossils show that these groups lived long ago. Some scientists think the first deuterostome was a worm. It may have lived on the ocean floor.

Some animals belong to a very special group called deuterostomes.

This group is part of a larger family called Bilateria. Bilateria are animals that have a body with two matching sides. They also have three distinct layers of cells. Deuterostomes are very important to study to understand life. They share a common ancestor that likely lived long ago. This ancestor might have been a worm-like creature. It probably lived on the ocean floor and used gill slits to breathe.

These animals grow in a very specific way during development.

When an embryo starts to grow, it forms a tiny hole. This hole is called a blastopore. In deuterostomes, this first opening becomes the anus. The mouth forms at a different spot later on. This is the opposite of protostomes, which develop their mouths first. Also, their body cavities form through a process called enterocoely. This happens when pouches of the gut pinch off to make a space.

Scientists have worked hard to organize these animals.

At first, scientists grouped them based on how they looked. In 1995, they changed the groups using DNA studies. DNA is the code inside living things. These studies showed which animals were truly related. This helped scientists realize that some animals were actually protostomes. Now, we know the main groups are Chordata, Echinodermata, and Hemichordata. There was also an extinct group called Cambroernida.

There are many different types of deuterostomes today. Chordates include many familiar animals like fish and humans. You can see this in sharks or bony fish.

Echinoderms are sea animals like starfish and sea urchins.

Hemichordates are often worm-like, such as acorn worms.

Fossils show these groups were well established 508 million years ago. This includes fossils like Pikaia, which was a primitive chordate.

Learning about deuterostomes helps us see how we are connected. Many of our own body parts have roots in this group. For example, chordates have a hollow nerve cord. This is a tube that carries signals through the body. Some hemichordates also have a nerve cord that looks like ours. Even the way our hearts pump blood is a very old trait. This trait likely came from a shared ancestor long ago. It shows how much life has changed over millions of years.

Deuterostomes are a major group of animals within the superphylum Deuterostomia. They belong to a larger clade called Bilateria, which consists of animals with bilateral symmetry and three germ layers.

This means their bodies can be divided into two matching halves. Deuterostomes are essential to biological study because they represent one of the two primary lineages of bilateral animals. Their sister clade is the Protostomia, which includes animals like ecdysozoans and spiralians. While protostomes develop their mouths first, deuterostomes follow a very different developmental path.

Understanding how a deuterostome grows requires looking at its earliest embryonic stages.

Every embryo begins as a zygote that develops into a hollow ball of cells called a blastula. In deuterostomes, the cells divide through a process called radial cleavage. This means the division planes are either parallel or perpendicular to the main polar axis of the embryo. Most deuterostomes also show indeterminate cleavage. This is a special type of development where the fate of early cells is not fixed. If the first four cells are separated, each can still grow into a complete, small larva. This process is actually what allows for the existence of identical twins.

During a stage called gastrulation, the embryo begins to form its internal structures. The first opening that appears in the embryo is called the blastopore. In deuterostomes, this blastopore becomes the anus and the cloaca. The mouth is formed later at a different location as the gut tunnels through the embryo.

Another defining feature is how the body cavity, or coelom, forms. Deuterostomes are often called enterocoelomates because their coelom develops through enterocoely. This occurs when pouches of the developing gut pinch off to create the body cavity.

Modern scientists classify deuterostomes into three main living phyla. The first is Chordata, which includes humans, fish, and tunicates.

This group is characterized by features like a hollow nerve cord. The second phylum is Echinodermata, which includes sea stars, sea urchins, and sea cucumbers.

The third phylum is Hemichordata, which contains worm-like animals such as acorn worms.

There was also an extinct group known as the clade Cambroernida. Within these groups, we find diverse creatures like the cartilaginous sharks or the bony fish of the clade Osteichthyes.

The way we group these animals has changed significantly over time. Initially, scientists used physical shapes to classify them. They included groups like Brachiopoda and Bryozoa in Deuterostomia based on how they looked. However, in 1995, researchers redefined the group using DNA molecular sequence analyses. This genetic evidence showed that some animals, like the lophophorates, actually belonged to the protostome group. Molecular studies also moved arrow worms from the deuterostome group to the protostome group. Genetic research has even found over 30 genes in deuterostomes that are not found in other animal groups.

The fossil record provides a deep history of these animals.

While some Ediacaran fossils like Burykhia are debated, others are more certain. Echinoderm fossils are very common starting from the third stage of the Cambrian period. We have found important early chordate fossils like Pikaia from the Mid Cambrian Burgess Shale. Other early fish fossils include Haikouichthys and Myllokunmingia from the Chengjiang biota. By 508 million years ago, fossils like Spartobranchus show that the main lineages were already well established.

Deuterostomes share many deep biological connections. Many members of Chordata and Hemichordata possess pharyngotremy, which is the presence of gill slits in the pharynx.

Chordates also possess a hollow nerve cord. Even the way blood is moved shows a shared history. Both hemichordates and chordates have a thickening of the aorta that acts like a heart. This suggests their common ancestor likely had these features. This ancestor was probably a benthic worm with a cartilaginous skeleton, a central nervous system, and gill slits.