For years, medical students were taught there are 78 organs in the human body.

In January, that number was revised with the announcement of a new organ called the mesentery.

Part of the digestive tract, the mesentery transports blood and lymphatic fluid between the intestine and the rest of the body.

Until recently, it was believed to be made of up separate structures.

But researchers in Ireland have revealed the mesentery is in fact one continuous structure, bringing the organ count for the human body up to 79.

So, what else don’t we know about the human body?

Healthline asked a few experts to share their theories on some long-held questions about our complicated machinery.

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Why do we have different blood types?

The major blood types are A, B, and O.

Type O is the most common blood type, and type AB is the least common.

Blood type can be determined by the types of sugars that stick to blood cells. Each individual has enzymes that are particular and will only add certain types of sugar to the blood cell.

Blood types can also be determined by the presence or absence of antigens and antibodies in blood and blood plasma.

But the reason why humans have different blood types in the first place remains a mystery.

Dr. Les E Silberstein, a spokesperson for the American Society of Hematology, and a hematologist at Boston Children’s Hospital and Harvard Medical School, describes it as an “enigma.”

“It is speculated that specific blood types and blood type antigens have evolved to fulfill a particular function,” Silberstein told Healthline.

“One theory is that people who evolved in a specific location may develop a specific blood type to protect them against diseases common in that area,” he added. “For example, the majority of African-Americans are group O, which is considered to be more resistant to certain forms of malaria.”

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Why do we have fingerprints?

Fingerprints have long been used as markers of human identity.

Fingerprint analysis forms an important part of forensic science and criminal investigation. Among other things, they can be used to identify individuals who are deceased.

The exact reason we have fingerprints has puzzled researchers for decades.

Roland Ennos, Ph.D., a professor of biological sciences at the University of Hull in England, says there are three potential reasons humans have fingerprints.

The first is that fingerprints help with touch sensitivity.

“Touch receptors are found in the ridges [of fingerprints] and these can amplify strains in the skin, so increasing sensitivity. But this could be a secondary use. Many other areas of our hands and feet, the palms and soles also have similar ridges, and we don't use them for sensitive touch,” he told Healthline.

The second possibility is that fingerprints help improve grip, though Ennos is unconvinced this is the case.

“The problem with this is that the friction of soft, rubbery materials such as skin rises with the area of contact. Fingerprints will reduce contact area, so should actually reduce friction on smooth surfaces. However, they could increase friction on rougher surfaces by getting into the troughs of the surface,” he said.

The third possibility is that the ridges on fingerprints could help prevent blisters.

“Blisters tend to occur when shoes rub on areas of the feet which don't have ridges, such as the heel and tips of the toes, but it's hard to prove,” Ennos said.

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Why do we yawn?

Have you ever found yourself needing to yawn directly after someone else has?

Robert Provine, a neuroscientist, and professor of psychology at the University of Maryland, and author of the book “Curious Behavior: Yawning, Laughing, Hiccupping, and Beyond,” says contagious yawning is a basic, neurologically programmed social behavior.

“Yawning may be a primitive form of empathy that binds tribal members together and coordinates their physiology, such as synchronizing arousal and bedtimes. Diminished contagion, whether from yawning or laughing, may be a novel measure of social disorders as in autism or schizophrenia,” he told Healthline.

But why do we yawn in the first place?

“The best starting point to this question is considering when we yawn,” Provine said.

“We yawn most just before bedtime, after waking, and when becoming bored. All of these situations involve ‘state change’ — shifting from one condition to another, whether from sleep to wakefulness, wakefulness to sleep, or alertness to boredom. Also, it has been suggested that yawning may cool the brain. Yawning is a vigorous, body-wide act that is ideal for stirring up our physiology and facilitating these shifts.”

It’s not just humans that yawn.

For most vertebrates (animals with backbones) yawning starts near the end of the first trimester of prenatal development.

Contagious yawns also occur in highly social animals such as chimpanzees and animals that travel in packs, like dogs.

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What causes us to laugh?

If you’ve ever heard someone laugh so hard they begin to sound like a chimpanzee, you have evolution to thank for it.

If you tickle a chimpanzee, its laughter sounds like panting.

“The sound of laughter came to symbolize the physical play that triggered it,” Provine explains. “Laughter indicates, ‘This is play, I'm not attacking you.’ The ancestral primate ‘pant-pant’ evolved into the human ‘ha-ha’ that involves a chopped exhalation, as in speech.”

The idea of “play” has evolved for humans from rough play of the chimpanzees to conversation and other high-order cognitive stimuli.

But laughter still remains highly social.

“We laugh thirty times more often in social than solitary situations. When alone, without the vicarious social stimuli of media and our imagination, laughter almost disappears,” Provine said. “Want more laughter in your life? Spend more time in playful encounters with friends, family, and lovers.”