Evolution of biological complexity

Evolution of biological complexity

The evolution of biological complexity is an important part of how living things change over time. Evolution has created very complex living things. But it is hard to say exactly how to measure complexity in living organisms. Scientists look at things like the number of cell types, morphology, or the genes an organism has.

In the past, many biologists thought that evolution was progressive (orthogenesis), meaning it moved in one direction toward what they called "higher organisms." But now, most scientists think this idea is not right. They believe natural selection does not have a set direction. Instead, organisms become more or less complex based on what their environment needs.

Even though the most complex organisms have become more advanced over time during the history of life, most organisms have stayed small and simple. The most common level of complexity has not changed much.

Selection for simplicity and complexity

Sometimes, creatures that can make lots of babies quickly have an advantage. This means they might become simpler so they can grow and make even more babies faster. For example, some germs like those that cause malaria can lose parts they don’t need because they live off another living thing.

But evolution can also make creatures more complex. This happens when living things keep changing to stay safe, like how our bodies fight off germs. These germs also change to try to hide. Over time, both the living thing and the germ get more complex as they try to outsmart each other.

This back-and-forth can make all kinds of living things — like animals, plants, and bugs — become more complex together as they deal with each other in their world.

Types of trends in complexity

Many people used to think that evolution always made living things more complex. But this isn't proven true.

Another idea is that complexity can grow without any special plan. If changes happen randomly and there's a basic level of simplicity that can't be gone below, the average complexity of all living things might slowly rise over time. This happens because there are more variations, but the most common level of complexity stays about the same. Only a few living things are very complex, while most are simple.

In this idea, it looks like evolution is moving toward more complex life only because we notice the few large, complicated organisms. In fact, most life on Earth is made up of tiny, simple creatures called prokaryotes. There are many more of these simple organisms than there are of more complicated ones, which makes large life seem more common than it really is.

Over time, the genes of living things have tended to become more complex. Some computer studies suggest that creating complex organisms might be a natural part of how evolution works. Proteins, which are important parts of all living things, also tend to change in certain ways over time. Sometimes, animals also get larger, a pattern known as Cope's rule.

Constructive neutral evolution

Further information: Constructive neutral evolution

Recent studies in evolution show that when there is less pressure to stay simple, organisms can become more complex. This happens through a process called constructive neutral evolution.

In this process, smaller groups of complex cells, like those in plants and animals, face fewer challenges to keep things simple. New genes can appear by chance, for example, when a gene makes an extra copy of itself. These extra copies aren’t always needed for survival, but they give the organism extra tools. Over time, if these tools become important, the organism now depends on them, which makes it more complex. This can also create new ways for existing genes to work together.

Scientists use this idea to explain how important structures inside cells, like the spliceosome and ribosome, gained extra parts over millions of years. It also helps explain how genes can change their functions and how some single-celled organisms developed unique traits.

Mutational hazard hypothesis

The mutational hazard hypothesis explains how living things can become more complex without this complexity helping them survive. It says that changes in the genes, called mutations, can sometimes add extra parts to the DNA that do not harm the organism. These extra parts are called non-coding DNA.

When groups of organisms become larger, random changes in their genes can allow more of these extra parts to stay. This can make the genes more complex over time. Scientists have studied many different plants and animals to understand how this works. They find that some have bigger genes with more of these extra parts, while others have smaller, simpler genes.

History

Further information: Orthogenesis

In the 1800s, some scientists thought nature always became more complex. They believed this because they thought the universe moved toward perfection.

Later scientists learned this wasn't true. They found that living things can become simpler or more complex, depending on what helps them survive. They also studied how order can develop in both living and non-living things, showing that these processes follow simple rules.