Non-cellular life

Some tiny things are not made of cells.

They are right on the edge of life. Viruses are one kind. They have a set shape. They can make more of themselves. But they need a host to do this.

Without a host, they cannot grow.

Viroids are even smaller. They are just tiny strands. They only infect plants. Some of these plants are used for food.

There are also things called obelisks. They were found in human gut bacteria. They are different from viroids. They can make proteins. These tiny things are very strange.

Most living things are made of cells. But some things are not. These are called non-cellular life.

Viruses are a well-known kind. They have a set shape. They also have genetic material. This is the code for life. Viruses can make more of themselves. But they need a host to do this. Without a host, they cannot grow. They cannot eat or use power. Some scientists think they are in a gray area. They sit between chemistry and life.

Viroids are even smaller than viruses.

They are just tiny strands of RNA. RNA is a type of genetic material. Viroids do not have a protein coat. They only infect flowering plants. They are so small that they do not make proteins. Some think they are relics from an ancient world.

Researchers also found things called obelisks. These were found in human gut bacteria. They are like viroids but they are different. Obelisks can make two proteins. We call these proteins oblins. They have a shape like a rod.

Most living things are made of cells. However, some things exist without a cellular structure. These are called non-cellular life.

Scientists used to say all life must have cells. Now, many people think other structures can be life too. This includes things that sit on the edge of being alive. They are often found in a gray area. This area is between chemistry and biology. It is a very interesting part of science.

Viruses are a famous type of non-cellular life. They have a set structure and genetic material. They can also build themselves from smaller parts. But viruses cannot live alone. They must have a host to work. Without a host, they cannot grow or eat. They also cannot make more of themselves. This is because they lack metabolism. They stay still until they find a host to use.

Viroids are even smaller than viruses. They consist only of short, circular strands of RNA. They do not have a protein coat. These tiny things only infect flowering plants. Some of these plants are important for business. A viroid genome is very small. It is only 246 to 467 nucleobases. In contrast, small viruses are 2,000 nucleobases. Some viroids act like tools called ribozymes. They can even cut themselves into pieces.

Scientists have many ideas about how these things started. In the 1980s, some people had a new theory. They thought viroids might be relics from an ancient world. This would be a world made of RNA. This idea became popular again in the 2010s. It helps explain how life grew from non-living matter. In 2024, researchers found something new called obelisks. These were found in databases of the human microbiome.

Obelisks are like viroids but they are different. They might live inside gut bacteria. Unlike viroids, they code for two proteins. Scientists call these proteins oblins. They also have a shape like a rod. There is also a idea called FUCA. This stands for the first universal common ancestor. It is a proposed non-cellular lifeform. It would be the ancestor of every living cell today.

Non-cellular life refers to entities that exist without a cellular structure. Most living things on Earth are made of cells. Historically, scientists believed that an organism must have cells to be considered alive. However, modern scientific criteria allow for different structural arrangements. These entities often exist in a gray area between chemistry and biology. They sit on the border between living organisms and nonliving molecules.

Viruses are the most well-known type of non-cellular life. They possess a defined structure and contain genetic material. Viruses can also spontaneously assemble from their own constituent parts. Despite these traits, they cannot perform life functions alone. They lack metabolism, which is the process of using energy. They also cannot grow or reproduce without a host. Because of this, many scientists view them as being at the border of life.

Viroids are even smaller than viruses and are even further from being living organisms. They consist solely of short, circular, single-stranded RNA. Unlike viruses, viroids do not have a protective protein coat. They are only known to infect flowering plants. Some of these plants are of commercial importance to humans. The genome of a viroid is extremely small. It ranges from only 246 to 467 nucleobases. In comparison, the smallest infectious viruses are about 2,000 nucleobases in size.

Viroids use a unique mechanism to make copies of themselves. They hijack an enzyme in the host cell called RNA polymerase II. Normally, this enzyme helps synthesize messenger RNA from DNA. Instead, the viroid uses it to perform "rolling circle" synthesis. This process uses the viroid's own RNA as a template to create new RNA. Some viroids also act as ribozymes. This means they have catalytic properties that allow for self-cleavage. They can cut and join their own genomes together.

Scientists have developed interesting theories about the history of these agents. In the 1980s, a theory emerged regarding the origin of viroids. Some researchers proposed they are "living relics" from an ancient RNA world. This hypothetical world existed before the evolution of DNA or proteins. This idea regained popularity in the 2010s. It helps scientists explain abiogenesis, which is the evolution of life from inanimate matter. These small molecules might represent crucial intermediate steps in that process.

In 2024, researchers announced the discovery of new RNA-based elements called obelisks. These were found within sequence databases of the human microbiome. It is possible that obelisks are hosted inside gut bacteria. Obelisks are similar to viroids but have distinct differences. For example, they code for two specific proteins called "oblins." They also have a predicted secondary structure that looks like a rod. This discovery adds a new layer to our understanding of non-cellular elements.

Another concept in this field is the First Universal Common Ancestor, or FUCA. FUCA is a proposed non-cellular lifeform. It is thought to be the earliest ancestor of all currently living cells. It would also be the ancestor of the last universal common ancestor and its sister lineages. Studying these proposed ancestors helps scientists map the history of all life. It connects the smallest non-cellular elements to the complex cells we see today. This research helps bridge the gap between simple chemistry and complex biology.