Evolution of biological complexity

The evolution of biological complexity is one important result of the process of evolution. Evolution has created some very complex living things. However, it is difficult to say exactly how to measure complexity in living organisms. Scientists have suggested looking 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 a certain direction toward what they called "higher organisms." But now, most scientists think this idea is misleading. They believe natural selection does not have a set direction. Instead, organisms become more or less complex depending 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, organisms that can have lots of babies quickly have an advantage. This means they might become simpler so they can grow and make 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 organisms 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 in the past thought that evolution had a built-in direction that always led to more complex living things. But this idea isn't supported by clear evidence.

Another way to think about it is that complexity can grow without any special plan. If we imagine that changes in complexity happen randomly and there is a basic level of simplicity that can't be gone below, then over time the average complexity of all living things might slowly rise. This happens because there are now more variations, but the most common level of complexity stays about the same. Only a tiny part of all 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 suggest that when pressure to stay simple is reduced, organisms can become more complex through a process called constructive neutral evolution. This happens because smaller groups of complex cells, like those in plants and animals, face fewer challenges to keep things simple.

In this process, 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, increasing its complexity. This can also create new ways for existing genes to work together in new ways.

Scientists have used 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 certain 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 suggests 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 without being removed. This can make the genes more complex over time. Scientists have studied many different plants and animals to understand how this works, and 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 that nature naturally tried to become more complex over time. This idea was influenced by other thinkers who believed the universe was moving toward a more perfect state.

Later scientists found that this wasn't true. They saw that some living things 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 basic rules.