Body plan

A body plan is a set of special features shared by many animals in the same group, called a phylum. For example, all animals with backbones, like humans and birds, share one body plan. But animals without backbones, like insects and spiders, have many different body plans.

We use the word "body plan" to talk about a kind of "blueprint" that includes things like whether an animal's body is symmetric, how its layers are arranged, if it has segments, where its nerves and limbs are, and how its digestive system works. Scientists who study how animals develop and evolve try to understand why different animals have such different body plans.

For a long time, people thought body plans appeared very quickly in Earth's history during a time called the Cambrian explosion. But now, we understand that body plans likely developed more slowly over time in the early part of Earth's history called the Palaeozoic. Scientists are also studying whether the ways animals and plants develop their bodies are limited by certain rules set by evolution.

History

Among early animal scientists, Linnaeus found two body plans besides animals with backbones; Cuvier found three; and Haeckel found four, plus eight more for a group called Protista, totaling twelve. Today, scientists recognize 36 groups of animals with different body plans.

In 1735, Linnaeus wrote a book called Systema Naturæ, where he grouped animals into categories like four-legged animals, birds, fish, and others.

In 1817, Cuvier studied animal bodies and split them into four main body plans, focusing on the central nervous system.

In 1866, Haeckel suggested all living things came from one origin and divided animals into groups with different body plans.

Gould later thought about how these body plans stay the same but changed his mind about this idea.

Origin

Many of the ways animals are built today began a long time ago during a period called the Cambrian. During this time, called the "Cambrian explosion," 20 out of the 36 main body plans of animals appeared. Some other body plans showed up much later, in a time called the Palaeozoic.

Even before the Cambrian, there were very old life forms with body plans that look different from animals today. These early life forms lived in a time called the Precambrian. The Cambrian explosion changed things a lot, replacing many of the older body plans with the ones we see now.

Genetic basis

Further information: evolutionary developmental biology and morphogenesis

Genes, embryos, and development work together to shape the body of an adult organism. This happens through complex processes called morphogenesis. Scientists study how genes control the development of body structures. They look at how important genes create special chemicals called morphogens. These chemicals spread through the body and tell cells where they are, which then activates other genes.

One big discovery is a group of genes called homeobox genes. These genes act like switches that set up the basic body plan in animals. These genes are very similar in many species, from fruit flies to humans. The simple body patterns in worms or fruit flies are related to the segmented spine in humans. The study of how genes shape body structures, called evolutionary developmental biology, is growing quickly, especially with detailed knowledge from studies on the fruit fly Drosophila.