Where did everyone go? How did Homo sapiens become the only remaining species of human?
Approximately 300,000 years ago, in a relatively short period of time in the context of evolution, there were at least nine different species of humans inhabiting the Earth. However, today, only one species, Homo sapiens, remains. This poses a significant question in the narrative of human evolution: what happened to the other species?
Professor Chris Stringer, who oversees the study of human origins at the Natural History Museum in London, suggests that the disappearance of certain species may have coincided with the migration of Homo sapiens out of Africa and into other parts of the world. However, it is uncertain if there is a direct correlation between the two events.
There are multiple hypotheses surrounding the vanishing of our human relatives, with only limited proof to fully understand the event. However, recent research is offering intriguing hints.
We are aware that around 40,000 years ago, H sapiens became the sole surviving human species among a wide and varied group of bipedal hominins. Theories vary, with some proposing that H sapiens had higher rates of infant survival compared to other hominins, or that changes in climate led other species to struggle. Other theories suggest a more aggressive involvement, such as H sapiens hunting or interbreeding with other humans and incorporating their genetic traits.
Approximately 300,000 years ago, the earliest populations of Homo sapiens began to emerge in Africa. While they did not resemble modern humans, they shared more similarities with us than other Homo species. These individuals had tall, round skulls with a nearly vertical forehead. They lacked the pronounced brow ridges seen in Neanderthals (Homo neanderthalensis) or the protruding jaw of more primitive species like Homo naledi. Additionally, they possessed chins, a characteristic unique to Homo sapiens (although the reason for this is unknown).
A recent publication in Nature challenged the belief that H sapiens evolved from a single location in Africa in one major evolutionary event. Through examining the genetic makeup of 290 individuals, the scientists demonstrated that H sapiens actually descended from a minimum of two separate populations that inhabited Africa for 1 million years before eventually merging through multiple interactions.
Experts in the study of ancient human origins are still engaged in heated debates about the exact identity of H Sapiens’ most recent ancestor, yet there is currently no definitive proof. Furthermore, the origins of H sapiens cannot be attributed to a single location. Fossilized remains found in Jebel Irhoud in Morocco, Omo Kibish in Ethiopia, and Florisbad in South Africa suggest that our species emerged from various regions.
The timing of when H sapiens migrated out of Africa is a topic of discussion. According to genetic evidence, there was a significant movement out of the continent between 80,000 and 60,000 years ago. However, this was not the initial migration. A puzzling H sapiens skull found in Apidima, Greece has been determined to be at least 210,000 years old.
There were multiple Homo groups living alongside H sapiens from 300,000 to 100,000 years ago. Some of these groups were similar to H sapiens, such as the stocky Neanderthals who adapted to Europe’s cold climate. The Denisovans, who lived in the high-altitude regions of Siberia and Tibet, possibly also existed in other areas.
Homo erectus, the long-legged “cosmopolitan” species – so called because of the impressive geographical range it spanned – still wandered through parts of Indonesia; Homo longi (also known as the “Dragon man”) lived in China. Homo rhodesiensis (also known as Homo bodoensis or Homo heidelbergensis – scientists continue to debate its name and membership) was alive in central and southern Africa.
Different types of animals were quite different from humans: H naledi, which had a small brain like an ape, roamed the grassy forests of South Africa, while the small Homo floresiensisand and Homo luzonensis inhabited and eventually perished on the islands of Flores and Luzon in Indonesia and the Philippines.
According to Professor Eleanor Scerri, leader of the human palaeosystems group at the Max Planck Institute of Geoanthropology in Jena, Germany, it is probable that hominin species were constantly facing extinction. It is uncommon that we have managed to survive.
The fossil evidence for most early human species is limited. For instance, H naledi remains have only been discovered at one location in South Africa. Other species are only represented by a small number of individuals. Interestingly, there are not many Homo fossils found in Africa, where H sapiens originated. According to Scerri, we still have a lot to learn about the other hominin species that coexisted with H sapiens in Africa.
There is a vast amount of information available on Neanderthals, including complete genetic sequences obtained from skeletal remains. These closely related beings inhabited Eurasia until approximately 40,000 years ago, living in small communities. Our knowledge about the Denisovans is considerably less, but what we do know has altered our understanding of human origins. In 2008, Russian archaeologists discovered several fragments of hominin bones, including a finger and part of a toe, in the Denisova cave in Siberia. The frigid climate had helped preserve some of the DNA in the finger bone, allowing for the sequencing of the entire genome of this previously unidentified species.
Based on the analysis of Neanderthal and Denisovan genetic data, it is believed that these ancient human species lived in small communities and often interbred. Mitochondrial DNA studies, which are passed down from the mother, indicate that the population of Neanderthals in Eurasia may have peaked at around 52,000 before decreasing. However, alternate estimates suggest that there could have been anywhere from 20,000 to 50,000 individuals at their height.
One significant benefit of our early human predecessors was their population size. According to Scerri, the smaller population sizes among Neanderthals and Denisovans led to more interbreeding, which is reflected in their genetics. This limited genetic diversity would have made these populations more vulnerable to diseases and less capable of surviving.
Compared to other species, H sapiens had larger social groups and a wider range of genetic diversity. This has implications that go beyond just being more resistant to diseases. According to Stringer, H sapiens also formed larger social networks that spanned across a larger geographical area. This was advantageous as it provided a safety net in times of environmental crisis, as individuals could move to areas where their distant relatives lived without fear of conflict. Additionally, these wide networks also facilitated the exchange of ideas and promoted innovation, according to Stringer.
The ability of H sapiens to adapt socially may have aided in their survival during climatic shifts that would have been fatal to less adaptable individuals and species. A study published in Nature in 2022 simulated the past climates and environments of H erectus, H heidelbergensis, and Neanderthals, and discovered that these species experienced significant decreases in their ecological niches before their extinction.
A more extensive simulation in 2023, encompassing six Homo species and the climate and vegetation of the last 3 million years, revealed that later Homo species had a greater capacity to thrive in a variety of environments, particularly H sapiens.
According to Prof Axel Timmermann, who co-authored this study and is the director of the IBS Centre for Climate Physics in Busan, South Korea, H sapiens emerged as the dominant species over Neanderthals, ultimately resulting in the extinction of the latter.
In a 2020 publication, he created a numeric representation to simulate the migration of H sapiens from Africa and incorporated available food sources. With this, he examined three potential explanations for the disappearance of Neanderthals: they were absorbed into H sapiens, there was a major environmental disaster, or H sapiens surpassed them in competition. According to Timmermann, only the latter possibility [competitive exclusion] can accurately explain the extinction of Neanderthals.
The researcher did not analyze the exact competitive advantage, but possible factors could have been superior tools, higher offspring survival rates, or potentially even group hunting, according to him.
Interbreeding human species
According to Stringer, there were several minor advantages that enabled H sapiens to surpass its relatives. While Neanderthals were also highly skilled, H sapiens may have had a slight edge. For example, seemingly insignificant developments like the creation of weaving and sewing needles (which were evident in the H sapiens fossil record 35,000 and 30,000 years ago, respectively) could have been the deciding factor in H sapiens’s success.
After learning how to weave, individuals have the ability to create baskets or snare nets. Using a sewing needle can also improve the insulation of tents, keeping babies warm and increasing their chances of survival. According to experts, larger social networks would have facilitated the sharing of these innovative techniques among H sapiens.
It is possible that Homo sapiens interbred with other species, as there is genetic proof of this occurrence. However, it is still debated whether this contributed to the extinction of these species. A small portion of individuals in Eurasia currently possess up to 2% Neanderthal DNA. In fact, some experts in genetics assert that they can reconstruct about 40% of the Neanderthal genome using the genetic sequences of living individuals.
In the meantime, the inhabitants of Oceania, including Australasia, Melanesia, Micronesia, and Polynesia, possess between 2% and 4% of Denisovan genetic material. Certain groups have an even greater proportion. Additionally, there is an intriguing puzzle surrounding a unidentified human predecessor who imparted 2% to 19% of their genetic makeup to present-day populations in western Africa.
In 2020, two scholars at the University of California, Los Angeles acquired the genetic makeup of over 400 individuals residing in Nigeria, Sierra Leone, and the Gambia. They approximated that early humans interbred with Homo sapiens in this area at some time during the past 124,000 years. This presents a significant philosophical question, according to Scerri. Did these early humans truly become extinct, or do they still exist in some form among us?
According to Prof Rebecca Ackermann, who co-directs the Human Evolution Research Institute at the University of Cape Town in South Africa, the classification of species is a matter of interpretation. This is a topic of intense discussion among paleoanthropologists, as some identify numerous species while others only recognize a few. In her opinion, the majority of these early human ancestors were likely not separate species, except for outliers like the small-brained H naledi. She believes that they should be considered regional variations rather than distinct species.
However, certain groups, regardless of whether they were a distinct species or not, certainly had more success than others. Our direct ancestors were among those who managed to survive. According to the experts I spoke with, this was largely due to chance and their actions. It is important for people living today to acknowledge this in order to overcome future challenges.
According to Stringer, networking and adaptability are crucial skills, especially in light of climate change. As a species, we will have to choose between cooperation and competition when faced with these crises. Looking at the history of Neanderthals and H sapiens, it is evident that the groups that cooperated more effectively were the ones that survived.