Humans vs birds: who is evolving more rapidly?

August 14, 2025

Related Topics:
Evolution,
Genetic variation,
Human evolution

A curious adult from Earth asks:

"I wonder for instance why for example the bird the robin stays so constant over time in the sense of evolution whereas humans evolve so rapidly."

What is evolution?

Evolution, or genetic changes in a group of organisms over time, was famously described by Charles Darwin in The Origin of Species (although the theory itself existed long before Darwin).1 What exactly do we mean by “genetic changes in a group of organisms?” Well, traits, such as height or hair color, can be passed on from parent to child in all living things. If some traits are passed on more often than others, then those traits will become much more common in a population over several generations. Similarly, if someone is randomly born with a new trait, and they pass on that trait to their offspring, and those offspring pass it on to their offspring, now a new trait has emerged in the population.

Evolution, the passing on of certain traits from generation to generation, does not happen by itself. It depends on other processes, which include natural selection, genetic drift, mutation, and gene flow. 

Natural selection is the process through which certain traits are more likely to be passed on over time because they increase a living being’s chance of survival or the likelihood that they will have offspring.

A green, rounded leaf with two caterpillars. One is green with black stripes while the other is black with few white stripes.
A green caterpillar is less likely to be seen and eaten by a bird than a black caterpillar. The color black will be selected against because it decreases survival. As a result, black caterpillars will not be able to pass on their traits to the next generation; this is an example of natural selection. (Image by N. Robles produced with biorender.com)

Genetic drift is the process by which a certain gene or genes that cause a specific trait become more common in a population due to random chance. This happens most often in small populations with little genetic variation.

Image of two groups of flies. The group on the left has one black fly and two orange-brown flies. The group on the right has one black fly and six orange-brown flies.
Let’s say that in a small population of flies, 1 out of 3 flies are black, while in a larger population of flies, 1 out of 7 flies are black. Based on random chance, the black color is more likely to be passed on in the small population than in the large population. This is an example of genetic drift. (Image by N. Robles produced with biorender.com)

Mutations are random, often rare, changes in a gene or genes. These can happen when our cells make a mistake or when they are damaged.

Image of two DNA sequences shown in gray and white shapes. The top sequence is ATGCGACT with the first C highlighted in blue. The bottom sequence is ATGGGACT with the second G highlighted in red.
Mutations can change small or large portions of a gene. In this example, only one base pair (C) was changed (to G). (Image by N. Robles produced with biorender.com)

Gene flow describes the movement of genes from one population to another, which can happen through migration or gene transfer. 

Image of one brown dog with an arrow pointing towards a group of five white dogs.
Let’s say that all dogs in a certain population have a white coat, and then a brown coat was introduced by a new dog from a nearby area. This would be an example of gene flow. (Image by N. Robles produced with biorender.com)

What changes the “speed” of evolution?

Evolutionary “speed” can be defined in many different ways. Is it how quickly a trait changes over time? If so, how much does the trait need to change? 

Is it rapid evolution if there are many young species rather than a few old species? What if the old species has many different populations that are rapidly changing but have not yet become their own species? 

What if the trait remains the same but the gene or genes that cause it have changed? We might not be able to observe this type of rapid evolution without a DNA analysis. 

How we define the “speed” of evolution depends on the type of question we would like to answer. However, there are some things known to affect how quickly a living being will evolve. For example: 

The strength of natural selection. If a trait has a strong effect on survival or reproduction, then it can quickly spread over a few generations.

On the left, a small white moth with many black spots. On the right, a black moth with many white spots. Both are set against a tan-beige background.
The Peppered Moth (Biston betularia) is a classic example of natural selection. The Peppered Moth exists naturally in two colors: white with black spots (left) and black with white spots (right). The white moth blends in more against the bark of the local trees while black moths are easily spotted and more likely to be eaten by birds, as a result black moths are rare. After industrialization in the mid to late-1800s, tree trunks in polluted cities began to blacken. This change in the moth’s environment favored the black moths and it quickly became the more common trait. (Images sourced from Wikimedia Commons, 2009)

The rate of mutation. Some species have higher mutation rates than others.2 The frequent introduction of new or different genes creates a higher probability that a population will change over time.

On the top left, a pink mitochondria with a beige inner membrane. On the top right, two sharks laying on white sand with rocks in the background. The sharks have a rounded head, barbels, and rounded fins. They are spotted with one large black circle outlined in white on their sides. On the bottom left, a single yeast cell is shown. It is rounded, brown, and some small organelles (mitochondria, vacuole, nucleus) are shown inside the cell. On the bottom right, a yellow bacterial cell with flagella (long squiggly extensions) on its sides and bottom is shown. Its DNA, in the center as darker yellow lines, is free-floating as bacteria have no nucleus.
Human mitochondria (top left) have their own genome! - with an approximate mutation rate of 1.91 mutations per base pair per year.2 The Epaulette Shark (top right) has the slowest mutation rate of any vertebrate (animals with backbones) at 7x10-10 per base pair, per generation.3 Yeast (bottom left) have an estimated mutation rate of 1.7x10-7 mutations per gene, per generation.4 The bacteria E. coli (bottom right) has a mutation rate of 1x10-3 per genome, per generation.5 (Images by N. Robles produced with biorender.com; epaulette shark image sourced from Wikimedia Commons, 2020)

The strength of gene flow. Like higher mutation rates, stronger gene flow can add new or different genes to a population.

A schematic shows a fish living in an isolated lake on the left, while on the right, fish in a river can freely interact with each other.
Imagine that there are two fish populations of the same species. One population is restricted to a large lake while the other is able to move in and out of their lake through a large river. The isolated population (left) relies on mutation for new traits to occur, spread, and ultimately lead to the evolution of the group. In contrast, the connected population (right) can mix freely with nearby fish, bringing in new traits more frequently and have the potential to evolve more rapidly. (Images by N. Robles produced with biorender.com)

Are humans evolving more rapidly than robins? 

The American robin (Turdus migratorius) is one of 86 currently recognized species of thrushes6. Thrushes, as a group, are estimated to be approximately 9.3 million years old, while the American robin is 3-4 million years old. In contrast, our closest relatives of the genus Homo are estimated to be approximately 2-3 million years old7 and Homo sapiens, or modern humans, are less than 1 million years old. It is important to remember that these are estimates! They are constantly changing as more fossils are found and technology improves.  

Similarly, it is difficult to know how many species of ancient humans there once were, given that modern humans are the only remaining survivors of this group. As of 2019, seven species of Homo have been described, although this number is highly contested! 

If each species of thrush evolved at the same rate, then a new species of thrush would have evolved every 100,000 years. If that were true for humans, then a new species of human would have evolved every 500,000 years. This would make thrushes a more rapidly evolving species than humans.

However, there is no standard evolutionary rate, even within a closely related group! This means that many species of robin may have evolved before 2 million years ago, or many species of human may have evolved less than a million years ago. To be able to tell these and other scenarios apart, we use genetic data and fossils to reconstruct the history of evolution. 

How different are ancient robins from modern robins? What about humans? It is rare for scientists to know the exact ancestor of any given living thing. Instead, they sometimes look at the oldest species within a group and assume that they are the most similar to their ancestor (although this is not always the case!). The Song Thrush (Turdus philomelos) is estimated to be the oldest thrush, and it differs from the American robin in feather patterns, beak color, and its home environment. Homo habilis is considered to be the oldest human and was much shorter, with a smaller brain case, than modern humans.8 

In the top left, a small brown bird with a lighter spotted chest. Next to the brown bird is a black bird with a red chest and orange beak sitting on a branch with a tree in the background. In the bottom left, a short, hairy, bipedal human with a protruding browbone and jaw. The title Homo habilis is located to the left with a green and white scale representing 40 cm and another representing 12 inches below it. In the bottom right, a tall mostly hairless modern human with a soft brow bone and jaw and a protruding nose. The title human (homo sapiens) male is located to the right with a green and white scale representing 40 cm and another representing 12 inches below it.
The Song Thrush (Turdus philomelos; top left) and the American Robin (Turdus migratorius; top right) belong to the same genus although they are distantly related, approximately 6 million years apart. The Song Thrush can be found in western Eurasia and northern Africa, while the American Robin, as its name may suggest, is only found in North America. Homo habilis (bottom left) and Homo sapiens (bottom right) are also part of the same genus and are about 1-1.5 million years apart. Fossils of Homo habilis have been found exclusively in eastern and southern Africa and it is unlikely that they ever lived outside of the African continent. Modern humans can be found anywhere on Earth. (Song thrush photo courtesy of Matti Virtala via Wikimedia Commons; robin photo courtesy of Rob Curtis, CC BY-NC-SA, via iNaturalist; human images sourced from the Encyclopaedia Britannica)

When observing the “ancient” robin and human, you’ll notice that the Song Thrush is more different from the American Robin than Homo habilis is from Homo sapiens, at least in coloration. Patterns and colors play a big role in the survival and reproduction of birds. The color of their feathers helps them find a mate and hide from predators. In contrast, modern and ancient humans largely differ in their anatomy. We have a larger skull, joints and bones that are better-suited for walking on two feet for long distances, and are taller which helped us move from forests to grasslands early on. These changes, particularly having more space for our large brains, have led to the mass migration of humans all over the world and our development of language, culture, and technology. 

So, if the “speed” of evolution is determined by change, then perhaps humans have evolved faster. After all, we have changed dramatically over only 1 million or so years compared to the six million years among thrushes. However, without an understanding of the genetics of these traits, it is not possible to determine whether some traits are easier to change than others, which would impact the “speed" at which we can see these changes happen. Ultimately, the “speed” of evolution relies on the details of our question - are we interested in one gene? A trait? A species? Or even an entire class of living things?

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Author: Nim Robles

Nim is a Ph.D. student in the Department of Biology, studying how hybrid incompatibilities influence speciation in Molly Schumer’s laboratory. Nim wrote this answer while participating in the Stanford at The Tech program.

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