Holozoa

Some tiny things are like animals.

They are very small. They live in the water. They help us learn about life.

These tiny things are called holozoans. They include all animals. They also include tiny relatives. These relatives are just one cell.

Some relatives live in the sea. One group eats tiny bacteria. They use a special collar to feed. This helps them stay alive.

Other relatives live on fish or humans. Some are very new to science. We are still learning about them.

Scientists look at their tiny parts. This helps us see how animals began. It is a big mystery.

Holozoa is a large group of living things.

It includes all animals. It also includes their closest relatives. These relatives are often just one single cell.

There are more than 1.5 million species in this group. Most of them are animals. Some are tiny protists. A protist is a single-celled living thing.

One group is called choanoflagellates. They are the closest living relatives to animals. They use a tiny collar to catch bacteria for food. This collar looks like the cells in a sponge.

Other groups have different ways of living. Ichthyosporea are often parasites. This means they live on or in other animals. They can live on fish or even humans.

Scientists study these tiny cells to solve a mystery. They want to know how one cell became many cells. They look at genes to find the answer. Many genes used by animals are already in these tiny relatives. This shows how animals began to grow.

Holozoa is a special group of living things.

This group includes all animals and their closest relatives. It also includes the common ancestor they both share. However, it does not include fungi. There are more than 1.5 million species in Holozoa. Most of these are multicellular animals. The rest are tiny, single-celled organisms called protists.

These small relatives help us understand how life changed over time.

How does this group work? Scientists look at how these organisms grow and eat. Choanoflagellates are the closest living relatives to animals. They are single-celled and use a tiny collar to catch bacteria.

This collar looks very much like the cells found in sponges. Other groups, like Ichthyosporea, often live as parasites. This means they live on or inside other animals like fish or humans. Some groups, like Filasterea, use thread-like parts to move. There is even a new group called Tunicaraptor with a special mouth.

Scientists have worked hard to map out this family tree. In 2002, researchers Franz Bernd Lang and Charles J. O'Kelly helped define Holozoa. They studied the genomes of two tiny organisms. One was a choanoflagellate named Monosiga brevicollis. The other was an ichthyosporean named Amoebidium parasiticum. This work showed that Ichthyosporea is a close relative to animals. Before this, many scientists thought some of these organisms were actually fungi. Now, we know they belong in the Holozoa group.

There are many important facts about these tiny creatures. The genome of Monosiga brevicollis has about 9,200 genes. This is much smaller than a human genome. For example, a human genome has about 2,900 million base-pairs. We also have clues from very old fossils. A fossil called Bicellum brasieri is about one billion years old. It might be the earliest known holozoan. Scientists also found fossils in the Doushantuo Formation. Some thought they were animal embryos, but they might be holozoan protists instead.

Studying Holozoa helps us see how big animals began. We used to think animals needed brand new genes to grow many cells. Now, we know that many animal genes were already there. For instance, some single-celled relatives have genes that help cells stick together. They also have genes for signaling, which helps cells talk to each other.

This means animals did not just invent new tools. They used old tools in new ways to build larger bodies. This discovery changes how we think about the start of animal life.

Holozoa is a biological clade that encompasses all animals and their closest single-celled relatives. This group is defined by its shared ancestry, specifically including all relatives of humans but excluding fungi. Together, these organisms represent more than 1.5 million species, most of which are heterotrophic, meaning they must consume other organisms for energy. While the vast majority are multicellular animals, the group also contains about 300 unicellular species known as protists.

Understanding Holozoa is essential for biologists because these single-celled organisms provide a window into how multicellular life first emerged.

The internal structure of Holozoa is divided into several distinct subgroups. The most famous group is Metazoa, which contains all multicellular animals. These organisms are characterized by a blastula phase during embryonic development and, in most cases, the formation of specialized tissues.

Below the animals are several lineages of protists. Choanoflagellata are the closest living relatives to animals, consisting of about 250 species that use a microvilli "collar" to feed on bacteria. Filasterea is a group of six amoeboid species that use thread-like pseudopods for movement. Other groups include the Ichthyosporea, which contains about 40 species that often live as parasites or commensals on fish and humans.

Recent discoveries continue to reshape our map of this group. The clade Pluriformea is a provisional group containing species like Corallochytrium limacisporum and Syssomonas multiformis. These organisms show a wide variety of shapes, ranging from amoebae to flagellates.

Even more recently, scientists discovered Tunicaraptor unikontum, a flagellate with a specialized mouth structure. The exact position of Tunicaraptor within the Holozoa family tree remains unresolved. It could be a sister group to Filasterea, a sister to the Filozoa clade, or even the most basal group of all Holozoa.

The history of Holozoa as a recognized group began in 2002. Researchers Franz Bernd Lang and Charles J. O'Kelly published a study in the journal Current Biology. They used phylogenomic analysis to examine the complete mitochondrial genomes of two specific organisms. They studied the choanoflagellate Monosiga brevicollis and the ichthyosporean Amoebidium parasiticum. This research proved that Ichthyosporea is a sister group to the clade containing animals and choanoflagellates. Before this, the relationships were unclear, and some Ichthyosporea were even mistakenly classified as fungi.

Genetics provides deep insights into the scale of these organisms. The genome of the choanoflagellate Monosiga brevicollis contains approximately 9,200 coding genes. It measures about 41.6 mega-base-pairs (Mbp), which is similar in size to the genome of some filamentous fungi. However, animal genomes are typically much larger. For comparison, the human genome is roughly 2,900 Mbp, and the fruit fly genome is about 180 Mbp.

These genetic differences help scientists trace how complex instructions for building bodies evolved over time.

Fossils offer a physical glimpse into the ancient past of Holozoa. The fossil Bicellum brasieri is a billion-year-old freshwater microscopic specimen. It is potentially the earliest known holozoan and shows two different cell types. Some researchers have also reinterpreted fossils from the Doushantuo Formation. While these were once thought to be early animal embryos, they may actually be non-animal holozoan protists. These organisms might have used palintomic cleavage, which is rapid cell division without growth, to spread and propagate.

Perhaps the most significant finding in Holozoa research is the concept of genetic co-option. For a long time, scientists believed that multicellular animals required entirely new genes to function. However, genomic studies show that many genes required for animal life were already present in single-celled relatives. For example, choanoflagellates and filastereans possess genes for cell-cell adhesion, such as cadherin and integrin. These proteins allow cells to stick together to form tissues.

Similarly, many signaling proteins and transcription factors used by animals are also found in unicellular holozoans. This suggests that the transition to multicellularity was not just about inventing new genes, but about adapting old ones for new purposes.