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 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, to 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 between 315,000 and 259,000 years old and come from both North and South Africa. The oldest remains are primarily skulls, but also lower jaws and other skeletal parts. Homo sapiens was probably widespread across Africa between 300,000 and 250,000 years ago.

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.

Evolution is the continuous adaptation of each species to the environment and circumstances they live in. Just like other species 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.
 

Neanderthals (Homo neanderthalensis) are a prehistoric human species who lived from 350,000 to 40,000 years ago in Europe, the Middle East and as far east as the Altai Mountains. Neanderthals are the closest relatives to our species Homo sapiens. We do not descend from the Neanderthals, but we share a common ancestor, the species Homo heidelbergensis. 

Homo heidelbergensis probably evolved from H. erectus in Africa around 600,000 years ago, and then spread to Europe. It is thought that H. heidelbergensis gradually evolved into Neanderthals (H. neanderthalensis) in Europe between 450,000 and 350,000 years ago. A little later, the African branch of H. heidelbergensis evolved into modern humans (H. sapiens), who emerged around 300,000 years ago in Africa.

Our species and Neanderthals share a common ancestor, but many people living today are also related to Neanderthals in another way. Since 2010 scientists have gradually uncovered the entire genome of the Neanderthals. It turns out, that between 1% and 3% of the genome in modern humans with roots outside Sub-Saharan Africa is derived from Neanderthals. Anatomically modern humans and Neanderthals met and interbred several times in the Middle East and Europe throughout the period between 100,000 and 40,000 years ago.

Considering how closely we are related, is it relevant to ask whether modern humans and Neanderthals are in fact the same species? The answer depends on how we 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 more robust 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 interbreed 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, as the existence of Neanderthal DNA in modern humans prove that the two did indeed interbreed successfully.

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."

Within science, explanations of natural phenomena begin as ”hypotheses”. Hypotheses are new ideas about and explanations of causes and relationships within a specific area of science, which is either little or not at all understood and tested. Hypotheses are thereafter examined and tested again and again by researchers within the area: can they explain everything within the subject, for example the evolution of new species or the presence of earthquakes. In fact, researchers across the globe try to make discoveries which show a hypothesis to be wrong – they are trying to refute it. And most scientific hypotheses usually end up refuted – researchers make one or more observations or studies, which show that the explanation does not work.

But a few, well-supported hypotheses manage the examination; they are tested again and again, but can still explain everything within their specific area. These solid hypotheses are elevated to theories. A kind of ”natural selection” take place with regards to hypotheses: only the most well-supported hypotheses survive. And even though an explanation have become a theory, researchers across the world continue to test them. For there is great scientific honor associated with proposing a solid hypothesis or theory, which can explain everything its specific area – and indeed also in being the researcher, who definitely disproves an established theory.

Blandt de teorier som har været afprøvet i lang tid og som har klaret sig hver gang findes Charles Darwins teori om evolution ved naturlig selektion og som forklarer udviklingen af klodens mangfoldighed af levende organismer, herunder menneskets evolution. Den har været underkastet videnskabelig afprøvning siden den blev fremsat i 1859 og har klaret alt, så den er nu enerådende i at forklare den store mangfoldighed af fortidige og nutidige organismer på Jorden og hvordan de er beslægtet med hinanden. Derfor kan vi være meget sikre på, at det er den rigtige forklaring.

Among the scientific, which have been tested for many years, and have stood the test every time, is Charles Darwin's theory of evolution by natural selection. It explains the diversity of living organisms on Earth, including the evolution of humans. It is and has been constantly subjected to scientific testing since it was proposed in 1859, and made it, so it remains the best explanation for the great diversity of organisms on Earth, past and present, and how they are related to each other. 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 currently oldest prehistoric DNA ever recovered is from 2 million years old sediments at Cape Copenhagen in North Greenland, and preserves DNA from the plants and animals who lived there in the past. The oldest preserved DNA from individual fossils derive from more than 1 million years old mammoth teeth discovered in Sibiria.

The Neanderthal bones and teeth from which DNA has been extracted all come from dry caves in the Altai and Caucasus Mountains, Belgium, Croatia, Germany and Spain, while Denisovans are found in caves in the Altai Mountains.

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. H. erectus existed until about 100,000 years ago, and some of them lived in cooler, northern areas with dry caves.
 

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 from 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 means shells, 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).

In very rare cases fossils contain DNA or protein molecules. One example is DNA from bones of the Denisovan humans as old as 200,000 to 50,000 years. The DNA was preserved because the bones were buried in sediments of a very cool and dry cave in the Altai Mountains. The same is the case for the pre-neanderthals and neanderthals found in caves in the Altai and Caucasus mountains, Belgium, Croatia, Germany and Spain from between 430,000 and 40,000 years ago. The cool and dry circumstances preserved the bones and reduced the degradation by microorganisms. 
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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.
 

Both no and yes. There are no "more important" species in our evolutionary history. The skulls in The Human Animal 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.

Some people consider apes "less evolved" than humans, but that is not the case. 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 Africa around 350,000 to 300,000 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.
 

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.

Several methods exist to find the age of a geological layer. 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 next to it.

Let us say, that two volcanic ash layers have been dated to 3.3 and 3.0 million years old, respectively. Later, fossil afarensis bones and teeth are found in a geological layer between the two ash layers. Thus, the fossils in the middle layer must be between 3.3 and 3.0 million years old.

If volcanic layers are not present close to geological layers, where fossils of prehistoric humans or apes are discovered, it is instead possible to date them by studying which other fossils of animals and plants are present in the same layers. Then it is possible to research if fossils of the same animals and plants are also present in other localities on Earth, and if their layers are close to volcanic layers (which can be dated using radioactive elements). As a specific animal or plant species only existed in one specific time period, it is possible to transfer the age of the dated volcanic layers, to the layers, which contain fossil remains of prehistoric humans or apes.

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, Central 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, fossil species from the important period between 9 and 6 million years ago are “missing”. This is the period when the prehistoric species lived, which became the ancestor of both humans and chimpanzees. Fossils from this period could shed light on the question of when and how the transition from quadrupedal to bipedalism began among the great apes. In the period between 6 and 4.4 million years ago, only two different species of Ardipithecus have been found, but several other species must have existed at the same time.

Completely new species are also emerging as paleontologists search both well-known and new and unexplored areas. In 2003, the first remains of Homo floresiensis (also nicknamed “the hobbits”) were found; a small, about 1 meter tall and 25 kilos heavy prehistoric human species that lived between 100,000 and 50,000 years ago on the island of Flores in Indonesia.

The Denisovans were first discovered in 2010 when DNA was extracted from a finger bone found in a cave in the Altai Mountains. The bone was thought to have come from a Neanderthal, but the DNA was so different that it must be a completely new species. Today (2025) only a few bone fragments and teeth are known from the Denisovans, who lived between 217,000 and 56,000 years ago in and around the Tibetan Plateau.

Since 2013, fossils of Homo naledi have been excavated from a cave complex in South Africa. The area was otherwise well-studied, but here several skeletons of another small human species were found, which grew up to 150 cm and weighed about 45 kg, and which lived between 335,000 and 235,000 years ago.

Between 2007 and 2015, scattered bone and tooth remains were excavated from Callao Cave on Luzon Island in the Philippines. In 2019, they were found to belong to a new human species, Homo luzonensis, that was clearly different from all other known species. H. luzonensis has been dated to between 67,000 and 50,000 years ago.

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.
 

This is a good question, which 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.

Currently (early 2025), we have roughly eight named and one unnamed species within the human genus Homo: H. habilis (2.4–1.6 million years ago), H. erectus (2.0–0.1 million years ago), H. heidelbergensis (700,000–200,000 years ago), H. neanderthalensis (350,000–40,000 years ago), H. naledi (335,000–235,000 years ago), H. sapiens (300,000 years ago and to the present), H. luzonensis (67,000–50,000 years ago) and H. floresiensis (100,000–50,000 years ago). The unnamed species is the Denisovans (217,000–56,000 years ago).

But there are uncertainties and disagreements – and perhaps many more species. Some researchers consider that Homo habilis belongs to the Australopithecus genus, or that some habilis fossils should be split off into its own species H. “rudolfensis”. Others consider that the African fossils of H. erectus are a separate species, called H. “ergaster”.

Fossil human remains from the Gran Dolina cave in Spain dated to between 1,200,000 and 800,000 years ago have been proposed as a separate species H. “antecessor”, but they also resemble H. erectus and early H. heidelbergensis. In Southeast Asia the situation is extremely complicated: there are numerous fossil human remains from the period between 700,000 and 130,000 years ago, but they are different parts of the skeleton and teeth that cannot be immediately compared. Two new species, H. longi and H. juluensis, have been proposed. H. longi may even be the first preserved skull of a Denisovan, but unfortunately cannot be directly compared to other finds of this species. Finally, some consider that H. floresiensis may belong to the Australopithecus genus, with which it shares many similarities, especially in the feet.

The earliest genus in the human lineage that is certain to have walked on two legs was Ardipithecus, which lived between 5.6 and 4.4 million years ago in East Africa. The skeleton shows that Ardipithecus' hip and hind limbs were adapted for upright, bipedal walking.

But its powerful, opposable big toe was also adapted for grasping branches and trunks, like in chimpanzees. Ardipithecus' upper body, arms and fingers were adapted for climbing trees. Studies of other fossils from the same layer as the Ardipithecus remains show that they lived in dense forests. Ardipithecus could walk on the ground, clamber in trees and walk on strong branches.

The next step in the human lineage was the gracile australopithecines, Australopithecus, between 4.2 and 1.9 million years ago. Fossil remains of hips and legs show that they were even better adapted to walking upright on two legs. They were naturally knock-kneed and had relatively short legs.

Fossil foot bones and footprints show that they still had relatively long toes and a large, opposable big toe and walked with fairly short steps. Their upper bodies, arms and fingers were adapted for climbing. The oldest fossils of Australopithecus come from areas with dense forest in the past, but the younger fossil finds show that these lived in many different environments from dense forest to open forest, savannah and grassland.

An 1.8-million-year-old foot of Homo habilis shows that they had fairly short toes and a curved arch, which was adapted for both walking and running. In Homo erectus, which appeared 2.0 million years ago, the modern walking and running skeleton emerged. Skeletons and traces on the bones show that H. erectus had relatively long legs, a narrower pelvis, very powerful buttock musculature and a long Achilles’ tendon.

The muscles of the head and shoulder girdle were separated, which made H. habilis and H. erectus worse climbers, but in return they could now swing their arms for balance. These are all adaptations for efficient long-distance endurance running at a steady pace of 10-12 km per hour. H. erectus became widespread, and their fossil remains and tools have been found in many different environments from Western Europe in the north, Africa in the south and China and Indonesia in the east.
 

“The Missing Link” is an outdated term that researchers have stopped using. It comes from a time when people were looking for fossils of those species that were exact intermediate form between two already known species. Or precisely the prehistoric species that was the ancestor of larger groups, such as all four-legged vertebrates, all whales or the entire human lineage. Researchers tried to find the “missing link” by first arranging known extinct and modern species in a chronological and evolutionary order. Then they worked out exactly which traits had changed in a specific order. And then it was a matter of finding the fossil that fit precisely into the series.

Today, the concept of “missing links” has been abandoned. The main reason is that researchers have become aware of how incomplete our knowledge of ancient species is. Many ancient species have left no traces at all because they lived in environments where their remains had no chance of being preserved as fossils. And if they have been preserved, then we usually only have a small part of the skeleton or skull from a single or few individuals. And if you do find a fossil that fits exactly as an intermediate form, you still can't be sure - it could be an individual that died before it had time to reproduce!

Even from places and times where there are enough fossils, it is clear that - just like in the present - many closely related species with small differences usually lived side by side at the same time. Then it is almost impossible to find the exact species that gave rise to the subsequent ones. For example, there were several contemporary species of Australopithecus around 2.5 million years ago. But because of very small differences between the Australopithecus species and their incomplete fossils, it is not possible to say exactly which one gave rise to the human genus Homo. Nevertheless, one can generally see that among the skeletons of the Australopithecus species, there are several anatomical features that support that one of them was the ancestor of the later human lineage.

Today, instead, kinship studies are carried out based on physical features or DNA, or both. Here, one maps out which species are most closely related to each other. Based on the phylogenetic trees and the “molecular clock”, one can roughly say something about when the last common ancestor of two species or an entire group lived. And make educated guesses about which anatomical features they had.

For example, we know that humans and chimpanzees are each other's closest living relatives because their DNA resembles each other more than that of any other species. Once upon a time, their ancestors belonged to the same, now long extinct, species of great ape. The difference in DNA between chimpanzees and humans arose after this species divided into at least two new populations, which evolved in different directions. By “calculating backwards” in the differences in their DNA, it is currently estimated (2025) that their prehistoric common ancestor lived between 9 and 6 million years ago. Fossils of this species have not been found, and it is uncertain whether they will ever be.