Bitter taste evolution

Many things in nature taste bitter. This bitter taste can be a warning. It tells animals that a plant might be bad to eat. Some animals, like sharks, can taste these things. Humans can taste bitter things too. Long ago, our ancestors ate many plants. This helped them learn which plants were safe. Later, people started using fire to cook. Cooking food makes it safer to eat. This changed how our bodies need to taste things. Our bodies are still changing today.

Many plants have toxins that can make an animal sick. Most of these toxins taste bitter. To stay safe, animals need to detect these bad tastes. This is why bitter taste is so important in nature.

Humans have a group of genes called TAS2R. These genes help us sense bitter things. Scientists have found these same genes in sharks, mice, and primates. In humans, these genes are found on chromosome 7 and chromosome 12.

Our bodies changed as our diets changed. Long ago, our ancestors ate many plants. This meant they had to be very careful. Then, humans began to use fire. Cooking food helps make it safe to eat. This meant we did not need to rely on taste as much. This change is called relaxed selective constraint. It means the body does not have to work as hard to keep a trait.

Because of these changes, our bitter taste genes keep changing today. Some genes even stop working. We call these broken genes pseudogenes. Even these broken genes can help control how we taste.

Bitter taste is a very important way for living things to stay safe. Many plants have toxins in their leaves that can make an animal sick. Most of these poisonous substances have a bitter taste. Having a sensitive sense of bitter taste gives an animal a big advantage. It helps them avoid eating plants that are not safe to eat. This survival skill is found in many different animals. Scientists have seen these bitter taste genes in sharks, rays, mice, and primates.

In humans, the ability to taste bitter comes from a gene family called TAS2R. These genes are located on chromosome 7 and chromosome 12. The genes work by using special parts called receptors. These receptors are found in taste buds, but also in the pancreas and the gut. When a bitter substance like phenylthiocarbamide (PTC) touches these cells, it starts a chain reaction. This reaction causes a fast increase in calcium ions inside the cells. This process tells the body that a bitter substance is present.

Scientists have studied how these genes changed over a long time. They found that these genes evolved before humans even moved out of Africa. In some primates, these genes changed through events called duplication. This means a gene is copied to make a new one. This happened in humans, chimpanzees, gorillas, orangutans, rhesus macaques, and baboons. By looking at these changes, researchers can see how different environments shaped our ancestors.

Diet plays a huge role in how strong these bitter taste genes are. Animals that eat mostly meat, like carnivores, often have a higher sensitivity to bitter tastes. This is because they do not rely on plants for food as much as grazers do. In humans, some genes have become "pseudogenes." A pseudogene is a gene that has been silenced and does not work the same way. Some of these broken genes still help control how we respond to taste.

Our ancestors' lives changed how these genes work today. About two million years ago, early humans began eating much more meat. This meant they encountered fewer toxic plants. Then, humans started using fire about 800,000 years ago. Cooking food helps make it safe and removes toxins. This change is called relaxed selective constraint. Because food became safer, the body did not have to rely on bitter taste as much. This is why our bitter taste genes continue to change even now.

The evolution of bitter taste receptors is a dynamic process in biology. This adaptation helps many species identify toxins found in plants. Most poisonous compounds in nature have a bitter taste. A sensitive palate provides a survival advantage to animals. It prevents them from ingesting dangerous substances. This trait is found in many vertebrates, including sharks, rays, mice, and primates. Scientists study these genes to understand how species adapt to their diets.

In humans, the ability to detect bitterness comes from the TAS2R gene family. This family is encoded on chromosome 7 and chromosome 12. There are 25 functional loci, which are specific locations on the genes, and 11 pseudogenes. A pseudogene is a gene that has been silenced and no longer functions normally. These TAS2R genes encode alpha subunits of G-protein-coupled receptors. These receptors are involved in intracellular taste transduction, which is the process of turning a stimulus into a signal. They are found in taste buds, the pancreas, and the gastrointestinal tract.

The mechanism of taste transduction begins when bitter compounds touch certain cells. A famous example of a bitter compound is phenylthiocarbamide, or PTC. When PTC is exposed to endocrine or gastrointestinal cells, it triggers an intracellular cascade. This is a chain reaction inside the cell. This cascade causes a large and rapid increase in intracellular calcium ions. This chemical change allows the body to recognize the bitter substance.

Dietary needs shape how these genes evolve. Species that eat mostly meat, known as carnivores, often have higher sensitivity to bitter tastes. This is because they can afford to reject plants more easily than grazers. Studies using quinine hydrochloride show that sensitivity is highest in carnivores, then omnivores, and lowest in grazers and browsers. This confirms that toxic plants are the primary selective force. Scientists measure this using the Ka/Ks ratio. This ratio compares synonymous mutations to non-synonymous mutations. In the TAS2R family, the ratio is over one for the parts of the receptor that bind to bitter ligands. This indicates positive selective pressure on those specific parts.

Genetic history shows that these genes changed long ago. The TAS2R genes evolved before humans migrated out of Africa. In primates like chimpanzees, gorillas, and baboons, these genes often change through duplication events. This is when a gene is copied to create a new one. Scientists also see high variety in these genes among primates and rodents. This suggests that while selective constraint exists, its effect is sometimes slight.

Human history is written in these genes. The highest diversity in TAS2R pseudogenes is often found in African populations. However, two specific loci, TAS2R6P and TAS2R18P, show more diversity in non-African populations. This suggests the functional versions of these genes existed before the migration out of Africa. In other areas, the environment allowed these genes to become silenced. This is called relaxed environmental constraint. Another gene, TAS2R16, shows different patterns. The 172Asn allele is very common in Eurasia and among African pygmy tribes. In Eurasia, this may be due to different plant knowledge that developed 10,000 years ago. In pygmy populations, the high rate might be due to genetic drift in small, isolated groups.

Lifestyle changes in hominids led to a phenomenon called relaxed selective constraint. This means the pressure to keep a gene working is reduced. About two million years ago, the hominid diet shifted toward more meat. This meant ancestors encountered fewer toxic plants. Later, humans began using fire about 800,000 years ago. Cooking food helps detoxify it. This reduced the need for the TAS2R system to detect poison. We see similar patterns in chimpanzees. Their diet is 85% ripe fruit, which rarely contains toxins. This is different from other primates that eat leaves and bark.