Frequently Asked Questions

The skulls in The Human Animal represent the prehistoric and contemporary great apes and human species for which researchers have the most fossil remains.

The chimpanzee is a modern species, so museums and collections worldwide have hundreds of skeletons and skulls. Its soft anatomy (skin, muscles, intestines, etc.) is therefore also very well known. On the other hand, fossil remains of chimpanzees are extremely rare. There is only a single find consisting of three teeth from Kenya, which are between 500,000 and 300,000 years old. The reason for the scarcity of chimpanzee fossils is that they live in dense forests. Forest animals rarely become fossils because the conditions for preservation on the forest floor are very poor. The forest floor does not protect the remains, and many scavengers quickly disassemble the carcasses and bones, after which wind, weather, and bacteria can easily decompose the bone and tooth remains.

Australopithecus afarensis is one of the best-known prehistoric species ever. Fossil remains from hundreds of individuals have been found in Tanzania, Ethiopia, and Kenya. Researchers have found remains from both adults and very young individuals. The fossils also show that there was a significant size difference between the sexes; males were up to 50% larger than females.

For Homo erectus, remains from over 50 individuals have been discovered. They have been found in many places around the world, from Western Europe in the north, Africa in the south, and as far east as China and Indonesia. This number also includes individuals of Homo ergaster from Africa, which some researchers, however, believe is a separate species. The most important discovery sites for H. erectus are Beijing, China (with finds of more than 40 individuals), and Dmanisi, Georgia (with skulls from five individuals).

Of course, there are plenty of complete skeletons of modern humans (Homo sapiens), both contemporary and from numerous archaeological finds. The oldest finds of modern humans are 195,000 years old and come from Omo Kibish in Kenya. Here, remains from at least four individuals have been found, including two skull caps, four lower jaws, parts of leg bones, and around 200 teeth.

Keep in mind that even when the number of individuals is mentioned, researchers usually only have parts of the skeleton or skull from each individual. Complete skeletons, where all the bones are preserved, are extremely rare. This is because predators, scavengers, and weather conditions in the past usually disassembled carcasses before the bones were buried and became fossils.

Researchers determine this by comparing fossilized bones with both healthy and diseased bones from modern humans and great apes. If the surface of the fossil bones resembles that of healthy modern bones, it is assumed that the fossil belonged to a healthy individual. Bones from diseased individuals are recognizable because they display shapes, defects, and changes similar to those found in modern diseased bones, distinguishing them from healthy ones.

It is actually extremely rare to find traces of disease in the bones of prehistoric species, but there are a few known cases. For example, a thigh bone from a Homo erectus individual dated to be between 1.0 and 0.7 million years old was found on the island of Java, Indonesia. This femur displays bony growths caused by a tumor-like accumulation of blood within the thigh muscles.

Another example is an elderly Neanderthal man who lived and died in France about 60,000 years ago. He was around 40 years old, had lost most of his teeth, and his bones displayed clear signs of severe, advanced arthritis. The discovery also indicates that other Neanderthals must have cared for him, as he would not have been able to survive on his own in such a weakened state. In fact, many fossilized Neanderthal skulls and skeletons show healed fractures and injuries. The pattern, location, and extent of these injuries are similar to those observed in modern rodeo riders, suggesting that Neanderthals sustained these injuries while hunting large prey in close combat.

However, in general, fossilized bones of prehistoric human and ape species show very few signs of disease. If prehistoric individuals suffered from a severe illness that could have left traces on their bones, they usually died from it long before it had time to leave a mark.

Humans are still evolving, and researchers have uncovered signs of rapid changes in the genome among populations in different parts of the world. We cannot say with certainty how we will look in a million years. But we can make an educated guess based on the changes currently taking place in our appearance.

One area where we see physical change in humans is the number of teeth in the mouth. It appears that we are in the process of losing our wisdom teeth, which are the rearmost molars. The standard number of adult teeth for Homo sapiens—and for other large apes like orangutans, gorillas, and chimpanzees—is 32 teeth. In the hunter-gatherer and early farming periods, worn-down molars show that Homo sapiens had a very rough diet. Back then, having a full set of adult molars was a significant advantage, as it provided more chewing surface for coarse food. Natural selection worked against people with genetic mutations that prevented them from developing all their adult molars, as they had less surface area to chew with, causing their teeth to wear down more quickly. Furthermore, all that chewing helped develop strong jaws. The muscles affected the growth and strength of the bones throughout life: the more muscle use, the stronger and longer the bones where the muscles attached.

But then, humans began to eat more and more cooked food. We also developed increasingly refined tools to cut and process food, making it softer and easier to chew. This required less chewing surface, and less chewing also meant smaller and shorter jaws. Suddenly, having fewer molars was no longer a disadvantage, and the mutations that reduced the number of wisdom teeth were no longer selected against. Perhaps the opposite was true since the energy the body used to form wisdom teeth could now be used for other things. Wisdom teeth have even become problematic in modern humans; consider how many adults have their wisdom teeth removed because they cause discomfort or because there is not enough space in the jaw!

Today, a significant portion of the human population is born without the development of one or more wisdom teeth, and they live their entire adult lives without issue. These people pass on the mutations that prevent the formation of wisdom teeth to their children. It is estimated that one-third of the population does not develop a full set of wisdom teeth. If this trend continues, future humans may have smaller jaws and, eventually, only 28 adult teeth in their mouths.

Recent studies of the human genome have revealed that between 300 and possibly up to 1,800 genes have been undergoing rapid change over the past 40,000 years. Different areas of the genome change at different rates in various parts of the world. Most of the genes involved have unknown effects on our physiology, but it is clear that they are changing.

For example, a gene variant that affects dopamine receptors in the brain has spread rapidly through European populations, although its specific effect remains unknown.

Similarly, several genetic variants that help the immune system fight malaria are spreading rapidly in African populations. Additional mutations that protect against malaria have also emerged and are spreading in Southeast Asia.

Today, people from different populations travel widely around the globe and intermix with each other. This means that advantageous genetic variants and mutations can now spread throughout the global population in relatively short periods—within a few generations or thousands of years.

But to create new human species, more drastic changes are needed. A million years is a long time, and much can happen. We can imagine a scenario where humanity has traveled into space and settled on other planets. Other planets would have different physical conditions than Earth, such as different atmospheric compositions, gravity, light levels, and light types. These conditions would change natural selection, influencing how evolution affects settlers. Selection pressures would act on different physical and genetic traits than on Earth. If the settlers become physically isolated from Earth's inhabitants, they may evolve in different directions and eventually become new species better adapted to their specific home planets.

This is a great question, and the answer depends on how you define a species.

The morphological species concept defines a species based on differences and similarities in appearance. According to this concept, Neanderthals and modern humans are two distinct species: Homo neanderthalensis and Homo sapiens. This is due to several significant differences in appearance: Neanderthals generally had stronger bones, larger braincases, a flatter forehead with prominent brow ridges, a longer back of the skull, larger nasal openings, and a receding chin, among other features.

The biological species concept, however, defines a species as a group of organisms that naturally mate and produce viable, fertile offspring. From this perspective, Neanderthals and modern humans could be seen as two subspecies of the same species: Homo sapiens neanderthalensis and Homo sapiens sapiens. This idea gained traction in 2010 when parts of the Neanderthal genome were successfully sequenced. The analysis revealed that between 1% and 4% of the genome of modern-day Europeans, Asians, and Native Americans comes from Neanderthals. This indicates that humans and Neanderthals interbred and had offspring together. It is believed that this interbreeding took place between 60,000 and 50,000 BCE in the Middle East during the second wave of human migration out of Africa.

Good question. In a way, we do descend from Homo erectus—but via another human species, Homo heidelbergensis. The reason we say we do not descend directly from H. erectus is that our species, Homo sapiens, resembles Homo heidelbergensis more than it does H. erectus. Therefore, H. heidelbergensis is more closely related to us than H. erectus.

Homo erectus evolved around 1.8 million years ago, probably in the Caucasus region, where the oldest fossils have been found at Dmanisi, Georgia. H. erectus then spread across much of Asia, Europe, and parts of Africa. The African fossils of H. erectus are called Homo ergaster by some researchers, but it is likely the same species.

Around 600,000 years ago, a new human species (Homo heidelbergensis) evolved from H. erectus in Africa. H. heidelbergensis then spread to Europe. It is believed that H. heidelbergensis evolved into Neanderthals (H. neanderthalensis) in Europe around 350,000 years ago. A little later, the African branch of H. heidelbergensis evolved into modern humans (H. sapiens), who emerged around 195,000 years ago in East Africa.

No, they wouldn’t. First of all, apes are not "less evolved" than humans. Every species of ape on Earth is just as evolved and adapted to its environment as we humans are to ours. Their evolutionary path simply took a different direction from that of humans because the environmental pressures — natural selection — were different in their habitats compared to the pressures that shaped human evolution.

Human evolution was shaped by a series of random events and changes in Earth's history. Global climate changes repeatedly altered the environment of East Africa, which, through natural selection, influenced the evolution of an upright, bipedal species of ape around 6 to 5 million years ago. This species branched into several new species, some of which went extinct while others evolved into even newer species. One species spread from Africa into Asia, evolved further, and parts of the population returned to Africa, where the evolutionary process continued. Eventually, modern humans evolved in East Africa about 195,000 years ago.

This long process will never repeat itself. The ancient species that initiated this process is extinct, and the sequence of events, climate changes, and random occurrences that influenced human evolution will never happen in the same way again. Therefore, no modern ape species will undergo the same evolutionary path that humans did.

However, if humans do not succeed in driving most apes and great apes to extinction, their evolution will continue. Climate and environmental changes will influence natural selection in modern species, and their evolution will head in new directions. This process will take hundreds of thousands or millions of years, but no one can predict where it will lead. Nor can we predict whether apes will develop intelligence or civilizations like humans.

When science describes something as a "theory," it actually means that researchers are very certain about it. The confusion comes from the fact that the word "theory" means one thing in everyday language and something quite different in science.

In everyday language, "theory" often refers to a vague, unproven idea about how something works or can be explained.

In science, "theory" refers to "the best, thoroughly tested, and well-supported explanation we have for a particular subject."

Scientific theories are constantly tested. Even Charles Darwin's theory of evolution by natural selection, proposed in 1859, has been tested ever since. It remains the best explanation for the diversity of organisms on Earth, past and present. Therefore, we can be very confident that it is the correct explanation.

We determine how closely related species are by comparing their DNA, as well as the shape of their bones and teeth. For living species, we can also compare internal organs, muscles, and skin. In general, the more two species resemble each other, the more closely related they are.

The process of determining relatedness is called phylogenetic analysis, which involves creating family trees to show how closely or distantly different species are related. This process is based on the principle that all modern species descend from ancient ancestors. Over the course of evolution, these ancestral species branched into different lineages and developed in different directions. Along the way, changes occurred in their physical appearance and DNA. These changes are reflected in modern species as distinct physical and genetic traits. If two modern species share certain physical and genetic traits, it indicates that they both inherited those traits from a common ancestor.

To determine relatedness, researchers compare the physical and/or genetic traits of the species in question with those of a "primitive sister group" — a group that has retained many of the original traits seen in their common ancestor. The sister group can be an extinct species or a modern one that scientists believe has retained many ancestral traits.

For example, if we want to determine the relatedness of modern vertebrates (mammals, turtles, amphibians, lizards, and birds), we could use modern bony fish as the "primitive sister group." This is because fossil evidence shows that all land-dwelling vertebrates descend from ancient fish that began walking on land about 360 million years ago. While modern fish have evolved over the past 360 million years, they have also retained several original features (like fins, gills, and gill covers) from that time. By using these ancestral features as a reference, scientists can compare the features of vertebrate groups to determine how closely related they are.

The principle of phylogenetic analysis is that the more shared "advanced" traits (those that differ from ancestral traits) two or more species have, the more closely related they are. If two species share several unique physical or genetic traits that are not found in other species, they are considered each other's closest relatives.

Similarly, scientists can conduct phylogenetic analysis using DNA and genes from different species. The more genes two species have in common, or the more similar their DNA is, the more closely related they are. For example, chimpanzees and bonobos are the living animal species whose DNA is most similar to that of modern humans. Overall, 98.89% of chimpanzee DNA is identical to human DNA, which shows that both species descend from a common ancestor — an ancient ape species. The genetic differences between chimps and humans emerged after this ancient species split into two lineages: one that led to the two chimpanzee species and another that led to modern humans.

Yes, this has been done, but only in the extremely rare cases where DNA has been preserved. Currently, we only have DNA from two prehistoric human species: Neanderthals and Denisovans.

The problem is that DNA molecules break down and degrade very quickly after a living organism dies. This process happens fastest in warm or humid climates, while very cold and dry conditions slow down the degradation. As a result, all prehistoric DNA found so far comes from cold, dry areas with permafrost or caves. The oldest prehistoric DNA ever recovered is from a 700,000-year-old prehistoric horse, preserved in the permafrost in Yukon, Canada.

The Neanderthal bones and teeth from which DNA has been extracted all come from dry caves in Croatia, Russia, Spain, and Germany, while the Denisovans were found in caves in Siberia and Spain.

Unfortunately, we will likely never be able to extract usable DNA from the human species that lived only in Africa and Southeast Asia because the climate there has been too warm and humid. Australopithecus afarensis is also out of the question, as the remains are simply too old, and any DNA would have completely degraded. However, there is hope of finding teeth or bones from Homo erectus that are well-preserved enough to contain DNA traces. The H. erectus species lived until about 140,000 years ago, and some of them lived in cooler, northern areas with dry caves.

The age of Australopithecus afarensis and other fossils is not determined by DNA. Instead, scientists determine the age of the geological layers in which the fossils are found.

One of the most precise methods is if volcanic layers are present above or below the layer containing the fossils. Researchers can date volcanic lava, ash, or tuff by measuring the content of radioactive elements in the layer, a process known as radiometric dating. Radioactive elements decay at a known rate, like a countdown clock. By measuring the ratio of certain radioactive elements to other elements, scientists can calculate the age of the volcanic layer and, by extension, the age of the layer with the fossils.

For example, if volcanic ash layers above and below a fossil layer are dated to 3.3 and 3.0 million years old, the fossils in the middle must be between 3.3 and 3.0 million years old.

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This is an excellent question, and researchers have not yet fully determined how long it takes. But they know that the process depends on the environment in which the remains are buried and that it takes between hundreds of thousands to millions of years for bones and teeth to turn into stone.

In Latin, "fossil" means "something that is dug up." However, in everyday language, fossils are usually understood as remains of prehistoric organisms that have been "fossilized"—that is, shells, plant wood, bones, and teeth that have been completely or partially turned into stone by mineral replacement.

For parts of an organism to become fossilized, the remains must be buried relatively quickly after death. If the remains are not buried promptly, predators and scavengers will tear apart the carcass and consume the soft tissues. Microorganisms, weather, and natural elements will then decompose any remaining soft tissues, followed by the breakdown of hard shells, bones, and teeth.

To protect the bones, they must be covered by new layers, such as clay, silt, or sand. In these buried layers, groundwater rich in dissolved minerals flows around and through the bones. The cavities inside the bones are filled with these new minerals. The groundwater also dissolves the original materials in the bones and teeth, which are then replaced with new minerals. For example, living bones are primarily composed of the mineral hydroxyapatite [Ca5(PO4)3(OH)], but in fossils, this is often replaced by minerals like calcite (CaCO3) and microcrystalline pyrite (FeS2).

Very young fossils that are only slightly fossilized or not fossilized at all are called "subfossils." They are usually only tens of thousands of years old. In extremely rare cases, traces of proteins or DNA molecules can still be found in subfossils. For example, in 2010, scientists extracted pieces of DNA from a 38,000-year-old Neanderthal bone from a cave in Croatia. The dry, cool conditions in the cave had preserved the bone and inhibited the activity of microorganisms. The oldest preserved prehistoric DNA so far comes from a 700,000-year-old prehistoric horse found buried in the permafrost in Canada.

But for most living organisms on Earth, their fate is total destruction. The soft tissues of a carcass—skin, hair, feathers, muscles, and internal organs—are consumed by carnivores, scavengers, and microorganisms. The remaining hard parts are then broken down by microorganisms, weather, and natural forces until nothing is left. In fact, it is estimated that only 1 in 100,000 living organisms dies under conditions that allow it to become a fossil. Organisms with hard parts like plant wood, shells, teeth, and bones have a better chance of being preserved than organisms with only soft tissues.

No, we cannot. In fact, we are quite certain that there are more species that have yet to be discovered, and researchers regularly find fossils of new species of prehistoric humans. When it comes to human evolution, the Middle East, Persia, Southeast Asia, and West Africa are relatively unexplored regions where exciting new fossil discoveries are very likely to be made, thereby enhancing our understanding of human evolution.

For example, there is a "missing" species in the timeline of human evolution between 2.0 and 1.8 million years ago — between Homo habilis (which lived in Africa) and Homo erectus (which appears to have originated in the Caucasus or western Asia). This unknown species is particularly intriguing because it represents the group that migrated from Africa to the Middle East, the Caucasus, and western Asia. What adaptations did this species have? Why did it leave Africa? And did other Homo species do the same?

Further back in time — long before the Homo genus emerged — we still haven’t found fossils that answer key questions about the evolution of bipedal walking. From around 6 to 4 million years ago, we only know of the genera Orrorin and Ardipithecus, which already show evidence of bipedal walking. Slightly older fossils could help reveal when and how the transition from four-legged to two-legged walking occurred among early apes.

New species are also being discovered as paleontologists search in new and previously unexplored areas. In 2003, for example, researchers found the first remains of Homo floresiensis (also called “the hobbits”), a small prehistoric human species that stood about 1 meter tall, weighed around 25 kg, and lived on the island of Flores, Indonesia, from about 95,000 to 12,000 years ago. In 2012, researchers identified yet another previously unknown human species (called “The Red Deer Cave People”) that lived between 14,500 and 11,500 years ago in southern China. This species displayed a unique combination of very ancient and very modern physical features.

These new species discoveries contribute to an increasingly complex picture of human evolution. It is now clear that human evolution was not a straightforward, linear process leading directly to anatomically modern Homo sapiens. Instead, human evolution can be better described as a bushy, branched network of species that followed separate evolutionary paths as they adapted to their local environments in Africa, Asia, and Europe, much like all other living organisms on Earth.

Both no and yes. There are no "more important" species in our evolutionary history. The skulls in The Human Beast were chosen because they are relatively well-known and widespread species that also show significant anatomical traits and important milestones in the history of the human lineage.

I won’t reveal which traits and milestones are represented (that can be discovered by examining your own measurement series on the skulls), but I will briefly explain why the individual species are included.

The chimpanzee is our closest living relative, and we can study its lifestyle in the wild today. Australopithecus afarensis is one of several Australopithecus species that lived more or less simultaneously in East and South Africa. The human genus Homo descends from one of these species, although it is still unclear which one. Homo erectus is perhaps the most successful species in the history of the human lineage: they existed for at least 1.7 million years and spread across Asia, Europe, and Africa. Studies of Homo erectus fossils and the discovery of their tools suggest they were behind several crucial inventions, behavioral changes, and cultural shifts in human history.

This is a good question, but it doesn’t have a precise answer. This is due to two factors: firstly, researchers disagree on how many distinct species actually exist, and secondly, new fossil finds of prehistoric human species are constantly being discovered. For instance, the Middle East and much of Asia are still relatively unexplored regions in terms of prehistoric human fossils, so new discoveries are almost guaranteed in the future.

At present, we can roughly count six named and two unnamed species within the human genus Homo:

• H. habilis (2.4–1.6 million years ago)
• H. erectus (1.8–0.1 million years ago)
• H. heidelbergensis (600,000–200,000 years ago)
• H. neanderthalensis (350,000–28,000 years ago)
• H. sapiens (200,000 years ago to the present)
• H. floresiensis (95,000–12,000 years ago)

The two unnamed species are the Denisovans (around 41,000 years ago) and the Red Deer Cave people (14,500–11,500 years ago in southern China).

However, there is still uncertainty and disagreement among researchers. Some argue that Homo habilis should actually be classified within the Australopithecus genus. Others suggest that the African fossils of H. erectus represent a distinct species, referred to as H. ergaster. Finally, there are those who believe that H. floresiensis might actually belong to the Australopithecus genus, as its feet, in particular, share many similarities with those of Australopithecus species.

One of the best examples of a significant evolutionary change in modern times can be seen in Pacific field crickets (Teleogryllus oceanicus) on the island of Kauai in Hawaii. A sharp increase in parasite numbers changed the natural selection pressures on the crickets. This change in natural selection, in turn, caused a significant shift in the appearance—physical and genetic variation—of male crickets, which in turn influenced sexual selection within the species.

Male Pacific field crickets attract females during mating season by making sounds. First, they produce a "calling song" to attract the female. Once the female has found the male, he produces a "mating song" that encourages the female to mate. The sounds are made by males rubbing their wings together: the wings have a ridge that creates the sound. The males also release pheromones from their exoskeletons.

For Pacific field crickets, it is the females who select their mates. The females are picky in their choice of mate: the quality of the male’s calling song, mating song, and pheromones are all crucial. Females prefer males that can produce calling songs and mating songs. They also prefer males whose pheromones signal that they are genetically different from the female. Within a population of male crickets, there is naturally some genetic variation in appearance and pheromones. Some males may, for example, have a genetic mutation that changes the structure of the wing, preventing them from making sounds.

Normally, in a population of crickets, natural selection favors males who can produce the right calling songs and pheromones that are different from the females'. It works against males who cannot produce the correct sounds or whose pheromones are too similar to those of the females.

But something changed on the Hawaiian island of Kauai. Biologists observed that from 1991 to 2001, the number of crickets on the island dropped dramatically. There were fewer and fewer calling males.

The reason for this decline was the arrival of a parasitic fly (Ormia ochracea) from North America. The fly uses the cricket's calls to locate them. The pregnant female fly lands on a calling male cricket and deposits its larvae on him. The larvae burrow into the cricket's body and eat it alive from the inside out. After a week, the larvae emerge as well-fed flies, leaving behind a dead cricket.

As a result, the population of crickets on Kauai changed from having an abundance of calling males to mostly silent males. The reason? A mutation had changed the shape of the wings, making them incapable of producing sound. The new "silent" trait spread quickly through the population. Females that were genetically predisposed to be "less picky" accepted silent males as mates, increasing their chances of reproduction compared to females that only mated with calling males.

The whole process happened biologically very quickly—between 12 and 20 generations of crickets from 1991 to 2001. It is a classic example of how natural selection (a change in the environment) drives changes in genetic variation (from winged, calling males to silent males) and ultimately shifts sexual selection (females that prefer silent males pass on more genes).