OUR TWO CLOSEST RELATIVES

 

Bonobos and Chimpanzees are our closest living relatives. Both of these species are members of the genus “Pan”. Together with humans, gorillas, and orangutans they are part of the family Hominidae (the great apes). Bonobos, formerly called pygmy chimpanzees, have the scientific name Pan paniscus. Chimpanzees (also known as common chimpanzees), on the other hand, have the scientific name Pan troglodytes.

The primate family tree shows how we relate to bonobos and chimpanzees, as well as other primates.

But how do we know bonobos and chimpanzees are our closest living relatives? Whole genome sequencing.

the primate family tree.png

WHOLE GENOME SEQUENCING

Your genome is your own unique sequence of DNA and is found in almost every cell in your body. Whole genome sequencing is the process of determining the complete DNA sequence of an organism's genome.

In whole genome sequencing, DNA is first sequenced by high-tech DNA sequencing machines. DNA sequencing machines cannot sequence the whole genome in one go. Therefore, scientists must break the genome into small pieces, sequence the pieces via the machines, and then reassemble them in the proper order to arrive at the sequence of the whole genome. It is this last part that takes the longest - reassembling and putting together this giant biological jigsaw puzzle that is the genome.

 

HUMAN, CHIMPANZEE AND BONOBO GENOMES

The DNA sequencing of the entire human genome was completed and published in April 2003, by the Human Genome Project. The Human Genome Project was a 13-year-long, international and publicly-funded research program initiated in 1990. The chimpanzee genome sequence was completed and published in 2005, two years later. Researchers found that the chimpanzee genome was 98.8% identical to the human genome, making them our closest living relative at this time. In June 2012, the bonobo genome sequence was completed. Researchers found that the bonobo genome was 98.7% identical to the human genome, thereby making both the chimpanzee and the bonobo our closest living relatives.

Kay Pruefer, one of the study researchers in the 2012 bonobo genome project, also added that, "There are certain parts of the human genome where you can see the humans are more closely related to bonobos than chimpanzees and other parts where they are more closely related to chimpanzees than bonobos. This is a much larger amount than we previously thought."

So, humans share 98.8% of their DNA with the chimpanzee and 98.7% of their DNA with the bonobo. Why then are we so similar to them, and yet so different?

 

WHY ARE WE SO SIMILAR TO BONOBOS AND CHIMPANZEES?

Human, chimpanzee and bonobo DNA is so similar because these three species are so closely related. We all descended from a single common ancestor species that was estimated to have lived around 5 to 6 million years ago (we have not yet found its remains). Humans and Pan (bonobo and chimpanzee) diverged from this common ancestor, giving rise to a human (Homo) lineage and a Pan lineage. The bonobo and chimpanzee species then separated from their common ancestor between 2 to 3 million years ago, which, as discussed earlier in “Speciation”, scientists believe was due to the Congo River. 

So as humans, common chimpanzees and bonobos gradually evolved from a common ancestor, their DNA, which was passed from generation to generation, changed too. In fact, many of these DNA changes led to differences in appearance and behaviour between humans, common chimpanzees and bonobos.

 

WHY ARE WE SO DIFFERENT TO BONOBOS AND CHIMPANZEES?

If human DNA is 98.8% the same with common chimpanzees and 98.7% the same with bonobos, why then are we so different?

As described earlier, the human genome is made up of over 3 billion nucleotides i.e. bits of information. Just 1.2% of this DNA (the difference between human and common chimpanzee DNA) equals to around 35 million differences. Some of these differences have a big impact, others don't. And even two identical stretches of DNA can work differently--they can be "turned on" in different amounts, in different places or at different times. So although humans, common chimpanzees and bonobos have similar DNA and many identical genes, they often use them in different ways. A gene's activity, or expression, can be turned up or down like the volume on a radio. So the same gene could be turned up high in humans, but very low in common chimpanzees.

For example, in humans, common chimpanzees and bonobos, the same genes are expressed in the same brain regions, but in different amounts. Thousands of differences like these affect brain development and function, and help explain why the human brain is larger and smarter than bonobos and common chimpanzees. A slight difference in our DNA compared to common chimpanzees and bonobos also means we can get infected by malaria, whereas they can’t.  

In the 5-6 million years since Humans and Pan (bonobo and common chimpanzee) diverged from our last common ancestor, chance mutations and natural selection have also changed each of our genomes in unique ways. At one point in the human evolutionary lineage, two of our chromosomes fused, leaving us with 23 pairs of chromosomes whilst the common chimpanzees and bonobos each have 24 pairs. This difference in the number of chromosomes also explains why we are so different.

So we have talked about some of the genetic similarities and differences between humans, common chimpanzees and bonobos, but what are the other (non-genetic) similarities and differences?

 

 BONOBOS, CHIMPANZEES AND HUMANS - A COMPARISON

 
comparative anatomy of bonobo chimpanzee and human.png
 

The infographic above gives an overview of comparative anatomy between chimpanzees, bonobos and humans, with a more in-depth comparison of anatomy, behaviour and habitat in the sections below.

 

GENERAL APPEARANCE

In terms of appearance, humans look quite different to bonobos and chimpanzees. Bonobos are smaller (average of 1.2 metres when standing) and more slender compared to the taller, stockier chimpanzees (average of 1.4 metres when standing). Bonobos have pink lips and black faces, whereas chimpanzees have brown lips and a pinkish face that darkens with age. Their fur is similar, being black, thick and covering the majority of the body (though bonobos have longer head hair with a central parting). However, these differences are small when compared with humans. 

Humans are taller than bonobos and chimpanzees (average of 1.6 metres for females when standing). Their skin tone and lip colour varies hugely, and they have hair rather than fur. The majority of hair is found on the head and in the pubic areas, and can also vary in colour. The arms of chimpanzees and bonobos are longer than their legs. In humans, however, their arms are about 30-40% shorter than their legs. Humans have a flat high forehead and face, no brow ridge, protruding nose and a chin, whereas bonobos and chimpanzees have a receding forehead, projecting face (prognathism), prominent brow ridge, flat nose and no chin. Humans also have eyebrows, whereas bonobos and chimpanzees do not.

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SOUNDS

Below are examples of bonobo and chimpanzee sounds. Bonobos are far more vocal than chimpanzees, using their voices to express themselves and to communicate with others. Bonobo voices tend to be relatively high-pitched and melodic, in contrast to the lower “pant hoots” of chimpanzees. There is not just one signature bonobo sound; they have a variety of different vocalizations for different situations.

How do you think they compare to the sounds we, as humans, make?

Bonobo sounds:

 
 
 
 
 
 
 
 

Bonobo sounds from: Bonobo Conservation Initiative

Chimpanzee sounds:

 
 

Chimpanzee sounds from: Chimps Inc.

SKELETON

Both the human skull and skeleton is very different compared to the bonobo and chimpanzee skull and skeleton. The images below shows some of the major differences between us (with chimp meaning both the chimpanzee and bonobo).

 
Image by:  School Bag

Image by: School Bag

Image by:  How It Works

Image by: How It Works

Bonobos and chimpanzees have much larger canines than humans. Although these allow them to kill and eat prey, they play a more significant role in defence. Male bonobos and chimpanzees also have larger canines compared to the females. In humans, the canines are a lot smaller and the same size in the two sexes. 

BRAIN

The brain of a bonobo and chimpanzee has a volume of 400cm3 on average. In contrast, humans have an average brain size of 1400cm3. Brain size alone, however, is not an absolute indicator of intelligence. There have been Nobel Prize winners with brains ranging from below 900cm3 all the way up to over 2000cm3. The structure and organization of the various parts of the brain is a better way of determining intelligence. Human brains have a high surface area because they are much more wrinkled than chimpanzee brains, with greater numbers of connections between many of its parts. These, as well as a relatively larger frontal lobe, allow us much more of the luxury of abstract and logical thought.

HABITAT

Humans are found all over the world and are able to live in drastically different climates. Bonobos and chimpanzees, however, are only found in Africa (in the wild). Bonobos can be found in the tropical forests of the Democratic Republic of Congo (South of the River Congo), while chimpanzees can be found in the tropical forests of West and Central Africa (North of the River Congo). 

DIET

Bonobos, chimpanzees and humans are omnivores. Most of the bonobo's diet consists of plants (fruit, flowers, leaves and seeds), but they may also eat insects or vertebrate animals up to the size of a small deer. Bonobos do not hunt cooperatively and they do not use tools to find/extract food. Chimpanzees also eat plants (fruit, flowers, leaves and seeds), but the males will also form organised hunting parties to hunt for meat. Chimpanzees will eat any animal smaller than themselves from insects to frogs, birds and small monkeys. In contrast to bonobos, chimpanzees use simple tools tools such as branches and stones to access hard to get food (nuts and termites/ants).

The human's diet is quite distinct compared to bonobos and chimpanzees. Although we are also omnivores, we consume much more meat and fat and less fibrous plant material. We also cook our food, which softens tough fibers, releases flavors, and speeds up the process of chewing and digesting. Cooked food also provides significantly more available calories than raw food. Through evolution, our dependence on heated food has had an impact on our anatomy. 

Compared to humans, bonobos and chimpanzees have much larger chewing muscles (masseter and temporalis), a stronger jaw and a bigger grinding surface on their molars to process leafy and fibrous plant material. Their enamel of their teeth is thin, an adaption to eating fruit. For humans, cooking food was a form of 'exterior digestion', so our jaws and jaw muscles became weaker and our teeth got smaller. Our enamel got thicker to help us grind up tougher foods and protect our teeth. Increasing the efficiency of our food and calorie intake meant that we were able to direct more energy to our brains and therefore develop large, more complex brains.

COMMUNITIES AND TERRITORIALITY

In the community of bonobos, females share power, exerting social dominance over the males. In the community of chimpanzees, males are dominant, with an alpha male leading the community and keeping his position through intimidation. Both the bonobos and chimpanzees' communities are made of multiple males and females and their offspring. With bonobos, mother-son and female-female bonds are very important, whereas with chimpanzees, male-male bonds are very important. Should conflicts arise in their community, bonobos resolve them with intercourse, whereas chimpanzees resolve them with violence.

Both bonobos and chimpanzees are territorial animals. However, territories of bonobos can overlap and bonobos tend to be friendlier with these other communities. Chimpanzees, however, are very specific about their territories, aggesssively patrolling their boundaries and avoiding neighbouring communities.