The Most Crowded Time in Human Evolution

By: Greg Schmalzel

As of March 2025, there are roughly 8 billion humans on planet Earth - all of which belong to the same species: Homo sapiens. Today, we are alone, the undisputed rulers of the planet. That said, there were many points throughout human evolution where the world was far less crowded with Homo sapiens, yet far more crowded with other human species.

Imagine walking through an ancient African landscape and encountering groups of individuals that, from afar look like you, but up close are entirely foreign. You find yourself in a world teeming with diversity: some humans with massive jaws built for chewing tough plants, others crafting primitive tools, and still others walking alongside them, adapted for entirely different ways of life. How would you react? Would you treat them kindly or confront them with nervous hesitation? 

This sounds like a hypothetical, Planet of the Apes-inspired scenario, but most it likely occurred throughout the 7 million years of hominin evolution. There were undoubtedly times when multiple, disparate human species were living at the same time. Some of these species were our direct ancestors. Others were evolutionary dead ends—close relatives who, despite their intelligence and adaptations, eventually vanished.

In this video, we’ll be taking a look at these other species and the point at which many of them existed simultaneously. We’re going to explore one of the most crowded times in human evolution—the moments when our family tree had the most branches. We’ll uncover the lost species that once walked the Earth, how they interacted, and why, in the end, only one species remains.

Us.

For the full video version, click HERE.

Speciation

Credit: S. V. Medaris / UW-Madison

One of the biggest misconceptions about human evolution is the idea that it followed a straight, unbroken path from apes to Homo sapiens. In reality, our evolutionary story is more like a branching tree—or even a river system, where different tributaries split, merge, and sometimes disappear entirely.

For much of history, scientists depicted evolution as a steady progression, where each species replaced the last. But as we uncover more fossils and study ancient DNA, we now know that multiple human species coexisted—sometimes for tens or even hundreds of thousands of years. Some went extinct, some interbred, and others continued evolving into new forms. This complex pattern is driven by speciation, the process by which new species form.

1. Geographic Isolation
When populations become physically separated—by mountains, deserts, or changing climates—they stop exchanging genes. Over thousands of generations, they adapt to their unique environments and eventually become distinct species. A modern example is the Kaibab and Abert’s squirrels, which were split by the formation of the Grand Canyon and evolved different traits over time.

2. Ecological & Behavioral Isolation
Even without physical barriers, species can diverge by adapting to different lifestyles. If one group of early humans relied more on hunting while another focused on gathering, they could evolve distinct physical and behavioral traits, leading to separate species. Homo habilis and Paranthropus boisei lived in the same environment, yet their diets and adaptations were vastly different—one developed tool use, while the other specialized in chewing tough plants.

3. Hybridization & Gene Flow
Evolution isn’t always about separation. Just as rivers sometimes merge, distinct species can interbreed, sharing genes and influencing each other’s evolution. This happened between Neanderthals, Denisovans, and modern humans, creating a tangled web rather than a simple lineage.

Although speciation has shaped human evolution, pinpointing exactly when and how it occurred is challenging. Fortunately, advancements in genetics and archaeology help us piece together this puzzle.

• Ancient DNA reveals when species diverged, interbred, and migrated, shedding light on the relationships between early humans.
• Fossils and tools tell us where different species lived, what they ate, and how they interacted with their environments.

By combining these methods, we’ve built a clearer timeline of our evolutionary past—one that shows human evolution as a dynamic, interconnected process rather than a simple march toward Homo sapiens. Now, let’s explore what this timeline reveals.

How Many Species Coexisted?

Homo sapiens and Co.

Credit: P. Plailly / E. Daynes

There were a few species that lived on earth alongside Homo sapiens since our inception 300,000 years ago.

The most commonly known species of these is surely Homo neanderthalensis. Neanderthals lived in Europe, the Middle East, and Central Asia from roughly 430,000 to 40,000 years ago, adapting to Ice Age conditions with stocky builds, large noses, and bigger brains than Homo sapiens. They crafted sophisticated tools, controlled fire, hunted big game, and possibly had a form of language. Neanderthals also showed symbolic behavior, buried their dead, and cared for the injured. I recently made a deep-dive video on Neanderthals, so I’ll leave a link to that in the description.

Denisovans were a mysterious species, and we just discovered them in 2010. Known mostly from DNA and tooth remains found in Siberia, they likely resembled Neanderthals but had unique traits, including genetic adaptations for high-altitude living. But, their exact physical appearance remains unknown since no complete skeleton has been found. They too had culture, with artifacts made of stone and bone found in Denisova Cave. They lived from approximately 3- or 400,000 years ago to 50,000 years ago.

Leaving the landmass of Eurasia, find a peculiar species that lived on the island of Flores in Indonesia. Here, Homo floresiensis, sometimes referred to as “The Hobbit” existed from 100,000 to 50,000 years ago. The name Hobbit comes from their small 3.5-foot statures. It’s believed that they experienced island dwarfism evolving through island dwarfism. Despite their tiny brains, they made stone tools, hunted pygmy elephants, and possibly used fire. Their disappearance may have been due to volcanic eruptions, climate change, or competition from Homo sapiens. Local legends of small, human-like creatures hint at possible ancient encounters.

On the island of Luzon, in the Philippines, Homo luzonensis lived between about 130,000 to 67,000 years ago. Like Denisovans, Luzonensis is a relatively recent discovery, so not a whole lot is known about this species. However, it had a mix of archaic and modern traits - curved fingers and toes suggest they were skilled climbers, possibly spending time in trees. Evidence of animal butchery indicates they hunted or scavenged, though little is known about their culture.

Homo naledi is estimated to have lived between 335,000 and 236,000 years ago, meaning they coexisted with early Homo sapiens. Their fossils were discovered in South Africa's Rising Star Cave. You may have seen lead archaeologist Lee Berger’s, Netflix documentary “Cave of Bones”, where he proposes these hominins possessed advanced symbolic thought, evidenced by cave wall carvings and possible burials. These claims are widely disputed and far from settled.

Lastly, we have Homo erectus, which was the oldest species that lived during the existence of Homo sapiens. These hominins were arguably the first true humans, walking on two feet full-time, advancing stone technology with their Acheulean handaxes, controlling fire, and leaving Africa for the first time. In fact, some species like Homo floresiensis may have been descendants of Homo erectus. The youngest remains of Homo erectus come from Java around 100,000 years ago. 

This brings us to about 7 different human species that shared the planet with our direct ancestors. Of course, the science is never settled, and further research may result in more species or the lumping together of some of these species.

Pre-Homo sapiens

Credit: Christian Jegou

But if we trace Homo erectus back to its origins, to a time before us, we reach a point when there could have been as many as 8 coexisting species including an entirely different genus. Homo erectus emerged in Africa around 2 million years ago. This marks a significant point in human evolution. Both because so many different hominins were alive and because it represents a transitional period from the older Australopithecine genus to our Homo genus. 

Some of the last remaining Australopithecines overlapped with early Homo species around 2 million years ago and we don’t know for sure which, if any, of the former evolved into the latter. But, these include Australopithecus africanus, boisei, robustus, and sediba. 

Australopithecus africanus lived in Southern Africa around 3.3 to 2.1 million years ago. They likely spent a lot of time walking bipedally but were still adapted for climbing with longer arms than legs. Their brains were slightly larger than their non-human apes and their hominin predecessors, but still much smaller than later humans.

Also living in South Africa around 2 million years ago were Australopithecus robustus and sediba. Robustus (sometimes referred to as Paranthropus robustus) is largely defined by its large, robust skull and face. It had massive premolars, a big face, and a well-developed sagittal crest, which is a ridge of bone that runs lengthwise down the middle of the top of the skull.  It’s something that appears on modern-day gorillas and acts as an attachment site for big, strong chewing muscles. Australopithecus sediba had a more gracile face and head with relatively smaller teeth. The Australopithecines were a genus with wide variation, but this distinction between robust and gracile forms is a common theme.

In East Africa, Australopithecus boisei (also sometimes referred to as Paranthropus boisei) lived from around 2.3 to 1.2 million years ago - making it one of the youngest species in the genus. Like the other robust species, it too had large posterior teeth that were probably used for eating hard foods like nuts, seeds, and fibrous tubers. Its flaring zygomatic, or cheek, bones, are another key feature of these robust species. 

In addition to these Australopithecine variants, we start to see an explosion in the Homo genus around 2 million years ago - starting with Homo habilis. Often called “the handy man,” it's considered the earliest member of our genus, coming on the scene a little before 2.5 MYA. Habilis is primarily found in East Africa, with fossil evidence from sites like Olduvai Gorge in Tanzania and Lake Turkana in Kenya. It had a relatively small brain (~600 cc), a more ape-like body with long arms, but a slightly more human-like face compared to Australopithecines. H. habilis is often credited with creating and using some of the earliest known stone tools—the Oldowan industry, consisting of simple, flaked stone tools for cutting and processing food. However, new evidence of stone tools from 3.3 MYA suggests a pre-Homo ancestor could have been the original handyman. 

Homo rudolfensis was another East African species coexisting with H. habilis. The two were very similar but Rudolfensis was a little bigger with a brain size of about (~750 cc). It had a longer face, and larger teeth, suggesting differences in diet and lifestyle. Some researchers debate whether H. rudolfensis is truly a separate species or just a variation of H. habilis. While no direct evidence of tool use is associated with them, their time period overlaps with the Oldowan culture, so it’s possible they participated in early toolmaking. 

We touched on Homo erectus earlier, but we also have Homo ergaster. Ergaster is often considered an African variant of Homo erectus, though some view it as a separate species. They shared many traits, including bigger brains, large forward-projecting jaws, and big brow ridges. However, these features seem to be more exaggerated in erectus.  H. ergaster had a more gracile build compared to its out-of-Africa counterparts. Like H. erectus, they used Acheulean tools and may have had basic control over fire. Are the differences enough to call this a separate species from Homo erectus or are they too similar? What do you think? Also, egaster mostly dates to around 1.8 MYA and after, but could we just not have unearthed earlier specimens yet? It’s a possibility. 

If so, this means as many as 8 different hominin species were living around 2 million years ago, and possibly more, encompassing 2 different genera. With all of these different populations of ancient humans, did they come into contact with one another? And if so how did it go down?

What happened when they met?

Credit: Troy Lawrence/The Varsity

Interbreeding: A Blended Legacy

Reconstructing interactions between ancient human species is a complex task. Were these encounters peaceful, violent, or somewhere in between? One of the strongest indicators of cooperation comes from genetics—revealing that interbreeding was a reality. While DNA from many ancient species remains elusive, we know that Homo sapiens interbred with Neanderthals and Denisovans.

In 2010, sequencing of the Neanderthal genome showed that non-African modern humans carry 1-2% Neanderthal DNA. Later studies found that Neanderthals also had Homo sapien DNA, proving gene flow was bidirectional. Some of these inherited genes provided adaptive advantages—such as the BNC2 gene influencing skin pigmentation in Europeans and the POU2F3 gene affecting keratinocyte function in East Asians, aiding survival in varied climates.

Denisovan DNA has also been found in modern populations, not just in Oceania but across East Eurasia and even the Americas. A 2024 study highlighted Denisovan-related adaptations in Papua New Guinea, where high-altitude populations carried genes linked to brain function (possibly for hypoxia tolerance), while lowlanders had genes associated with immune response, likely aiding resistance to malaria.

These findings reshape our understanding of human evolution as a dynamic, interwoven process—akin to a braided river where genetic streams merge and separate over time. Yet, while interbreeding suggests cooperation, it does not rule out conflict.

Violence and Competition: A Darker Side of Human Interaction

Violence is deeply embedded in human history. At Jebel Sahaba (Sudan), a 14,000–18,000-year-old cemetery provides early evidence of large-scale conflict. Over 60% of individuals bear signs of violent trauma, including embedded projectiles and fractures, suggesting sustained intergroup warfare.

But did violence extend to interspecies conflicts? One intriguing possibility comes from Shanidar Cave in Iraq, where Neanderthal remains dating to 50,000 years ago include an individual (Shanidar 3) with a rib wound. The injury appears to have been caused by a long-range, low-energy projectile—similar to weapons used by Homo sapiens but not known in Neanderthal technology. This raises the possibility that early modern humans engaged in lethal encounters with Neanderthals, potentially over resources, territory, or even mates.

While definitive evidence of interspecies warfare remains scarce, the combination of genetic exchange and possible violent encounters paints a nuanced picture of our past—one of both cooperation and competition as species vied for survival in an ever-changing world.

Conclusion: Why are we the only ones left?

Throughout history, as many as eight human species coexisted, likely crossing paths. Yet today, only Homo sapiens remain. Why?

For decades, researchers have explored this mystery. One answer lies in our advanced cognition—our ability to create sophisticated tools, art, and cultural practices. For instance, the complex projectile weapon possibly used against Shanidar 3 suggests early innovation in warfare or hunting.

Our social structures may have also played a key role. Unlike other hominins, Homo sapiens formed vast, interconnected networks that enabled the sharing of resources, knowledge, and innovations. Larger groups meant a broader gene pool, reducing harmful genetic traits and increasing adaptability.

Beyond scientific explanations, there’s an undeniable uniqueness to us. Despite being primates, we possess a capacity for abstract thought, imagination, and existential questioning.

Religions, found in every culture, may reflect this distinct human trait—our drive to transcend our biological limitations. No other species, as far as we know, contemplates its place in the cosmos or strives for meaning beyond survival.

Perhaps this very pursuit of something greater is what set us apart from other hominins. But why us? And where does this drive come from?

As you ponder these existential questions about our species, check out this video on animism, possibly the world’s oldest religion.

Sources: 

[1] Larsen, Clark Spencer. 2019. Essentials of Biological Anthropology. 4th ed. New York, NY: W.W. Norton & Company, Inc.

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[3] Liming Li et al. 2024. “Recurrent gene flow between Neanderthals and modern humans over the past 200,000 years.” Science 385(6705).

[4] Reilly, P., et al. 2022. “The contribution of Neanderthal introgression to modern human traits.” Curr Biol. 32(18):R970-R983.

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[6] D. Yermakovich, M, et al. 2024. “Denisovan admixture facilitated environmental adaptation in Papua New Guinean populations.” Proc. Natl. Acad. Sci. U.S.A. 121(26):e2405889121. 

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