Becoming Human Part 2

This is the second part of a post summarizing current understanding of the evolution of humans. In the previous post, I outlined the evolution of pre-human species from the first monkeys around 35 million years ago (Mya) to the appearance of the first human species around 2 Mya. This post takes a look across the evolution of humans from the appearance of the first human species Homo habilis to the appearance of anatomically modern Homo sapiens sapiens around 250 thousand years ago. The following figure summarizes the evolution of humans over the last 2 million years, based on [1] with some modifications to take account of some recent discoveries.

The evolution of humans (the genus Homo) over the last 2 million years. Updated from Figure in Wikimedia. User:Conquistador, User:Dbachmann / CC BY-SA. (https://creativecommons.org/licenses/by-sa/4.0)- . The late survival of robust australopithecines (Paranthropus) alongside humans until about 1.2 Mya is indicated in purple. The rapid “Out of Africa” expansion of H. sapiens is indicated at the top of the diagram, with admixture indicated with Neanderthals, Denisovans, and unspecified archaic African hominins.

Homo habilis (Handy Man)

Homo habilis

Homo habilis evolved from an Australopithecine species, quite likely Australopithecus afarensis, in South and East Africa in the late Pliocene or early Pleistocene, around 2.5 Mya. Louis Leakey gave homo habilis the nickname “Handy Man”, as this species used stone tools and tools made from animal bones. Handy Man was our first human ancestor, ie. the first member of the genus Homo. All members of the genus homo are referred to as “humans”. We are separated from the first humans by around 100,000 generations [2].

H. habilis was no more than around 1.3m tall and had disproportionately long arms compared to modern humans; more chimp-like and adapted for swinging and load-bearing. H. habilis had smaller molars and larger brains than the Australopithecines, and a less protruding face. H. habilis had a continuous bony brow ridge above both eyes and a brain size ranging from 550 cc to 680 cc, around 45% larger than Australopithecus.

Habilis lived in a predominantly grassland environment. The climate was becoming cooler and drier and this may have been the impetus for new feeding strategies that included scavenging and tool use. Chemical analysis suggests that this species was mainly vegetarian but did include some meat in their diet. H. habilis likely used tools for butchering meat which it scavenged from other carnivores. It is generally thought that the intelligence and social organization of H. habilis were more sophisticated than those of Australopithecines. Their larger brain size was associated with a larger cerebellum, particularly the frontal and parietal lobes which govern speech in modern humans. The timing of the origin of speech is difficult to determine, as speech organs don’t fossilize and its challenging to deduce speaking from broader anatomical or artefact clues [4]. Dates as early as 2 Mya and as late as 50,000 years have been suggested.

The communication skills (both vocal and gestural) of our closest living relatives — chimpanzees, bonobos and gorillas — are complex, and presumably (though not definitely) indicate the evolutionary platform from which hominin linguistic skills evolved. Presumably Australopithecines had even more complex vocal and gestural communication and selection for better vocalization may have been a factor in the increasing size of the cerebellum in H. habilis. It is generally thought that linguistic speech developed later in Homo erectus, or possibly Homo heidelbergensis. I think it is likely that H. habilis used a form of proto-speech involving more complex vocalization than Australopithecus.

It is thought that an isolated group of H. habilis evolved into H.erectus around 2 Mya and that H.erectus and H.habilis coexisted for another 0.6 million years until the extinction of H. habilis around 1.4 Mya

Homo erectus (Upright Man)

Homo erectus, female. Reconstruction based on ER 3733 by John Gurche

Homo erectus first appeared about 2Mya and were more similar to modern humans than to their ancestors the australopithecines, with a more humanlike gait, body proportions, height and brain capacity.

It is generally thought that linguistic speech developed later in Homo erectus, or possibly Homo heidelbergensis. It has been argued that a primitive form of speech co-evolved with skilled manual-motor skills involved in crafting the more sophisticated stone tools associated with the Auchelean culture that appeared around 1.75 Mya [4, 5]. Evidence has been found that H. erectus used fire by about 400 thousand years ago (kya), though claims for the earliest definitive use of fire for cooking, light and heat vary widely.

Homo ergaster

Homo ergaster or Homo erectus ergaster is an extinct homo species that lived inSouthern and Eastern Africa from about 1.9 Mya to 1.4 Mya. It was originally thought to be a separate species that evolved from H. habilis but is not mostly considered to be an early form, or an African variant, of H. erectus.

Humans migrated out of Africa several times during the last 2 million years. The first migration was around 1.9 Mya with Homo erectus (or the African variant of Homo erectus known as Homo ergaster) migrating into Asia and Europe, where in Europe they evolved into Homo antecessor around 800,000 years ago, and in Asia remained as Homo erectus (although some argue that Homo ergaster was a completely different species from erectus, so that it was ergaster that evolved into erectus in Asia).

Female H. erectus

Erectus ranged from Southern Africa all the way to modern-day China and Indonesia from about 1.9 million to possibly as recently as 1.4 -5.5 kya, making them the longest-lived species of Homo, having survived for over a million years. . There were several other contemporary groups of early Homo in Africa – including H. ergaster and H. rudolfensis – which were likely various stages in the evolution of H. erectus.

In addition to these early humans, researchers have found evidence of an unknown “superarchaic” group that separated from other humans in Africa around two million years ago. These superarchaic humans mated with the ancestors of Neanderthals and Denisovans, according to a paper published in February 2020 [6]. This marks the earliest known instance of human groups mating with each other—something we know happened a lot more later on.

Homo cepranensis and Homo antecessor (Pioneer Man)

Homo antecessor

These proposed species may be intermediate between H. erectus and H. heidelbergensis and possibly as the most recent common ancestor of modern humans, Neanderthals and Denisovans. H. cepranensis refers to a single skull cap from Italy, estimated to be about 800,000 years old. H. antecessor is known from fossils from Spain and England that are dated 1.2–0.5 Mya. In 2010, stone tool finds were reported in Happisburgh, Norfolk, England, believed to have been used by H. antecessor about 950,000 years ago – the earliest known population of the genus Homo in Northern Europe.

Homo antecessor was about 1.6-1.8 m tall, and males weighed roughly 90 kg. Their brain sizes were roughly 1,000–1,150 cm³, smaller than the 1,350 cm³ average of modern humans. Due to its scarcity, very little more is known about the physiology of H. antecessor, yet it was likely to have been more robust than H. heidelbergensis.

Female Homo antecessor

Researchers think that H. antecessor had the same development stages as H. sapiens, though probably at a faster pace. Other features acquired by the species are a protruding occipital bun, a low forehead and a lack of a strong chin. Homo antecessor may have had symbolic language and an ability to reason. Some fossil bones have cuts that indicate that H. antecessor may have practised cannibalism.

Very few fossil fragments of H. antecessor have been found, in particular no complete skull. Its existence as a separate species remains controversial, as does the claim that it is an intermediate species between H. erectus and H. heidelbergensis. A very recent paper published 1 April 2020 [7] analysed million year old proteins from the enamel of an H. antecessor tooth. Welker and co-authors conclude that antecessor split from our ancestors before the split that led to Neanderthals and Denisovans. That makes them a sister lineage to modern humans, not direct ancestors [8].

Homo heidelbergensis

Homo heidelbergensis

The population of Homo erectus/Homo ergaster that stayed behind in Africa evolved into Homo heidelbergensis. Heidelberg Man lived from about 800,000 to about 300,000 years ago. They were tall individuals who were 1.80 m tall, muscular and very strong (would reach 105 kg). Heidelberg man had large skulls averaging 1350 cm3 with a brain around 90-100% size of that of H. sapiens, flattened skulls compared to those of the present man, with outgoing jaws and large nasal opening.

Heidelbergensis was first human to hunt large animals (with wooden spears tipped with stone points) and to build shelters. Heidelbergensis used fire and its anatomy suggests that its speech abilities more closely resemble those of modern humans than chimpanzees [9-11]. It is argued that Heidelbergensis was probably the first species to have symbolic speech, but it is also possible that it may have only had a pre-linguistic system of communication. Evidence from a pit containing skeletons in Spain suggests that Heidelbergensis may have buried its dead.

Homo heidelbergensis

Homo heidelbergensis migrated out of Africa around 600,000 years ago. This species migrated into Europe, in which that population evolved into the Neanderthals, and in Asia where that population evolved into the Denisovans. It has long been thought that the heidelbergensis populations that stayed behind in Africa evolved into our species, Homo sapiens, between 300,000 and 200,000 years ago, as is shown in the diagram of human evolution near the beginning of this post.

Recent genetic studies have suggested that Heidelbergensis should be included entirely in the Neanderthal-Denisovan lineage and the divergence between this lineage and modern humans pushed back to before the emergence of H. heidelbergensis, to about 600 to 800 kya, the approximate time of the disappearance of Homo antecessor.

Neanderthals

Neanderthal man

Homo neanderthalensis, alternatively designated as Homo sapiens neanderthalensis, evolved from Homo heidelbergensis in Europe. Genetic analysis has produced dates for split between Homo neanderthalis and Homo heidelbergensis ranging from 315 kya to more than 800 kya [12, 13]. Mitochondrial DNA analysis suggests that the split between Homo neanderthalis and the Homo heidelbergensis who went on to evolve into Homo sapiens occurred 550 (±54) thousand years ago [14]. A recent analysis of dental evolution rates concluded that the split between Neanderthal and modern human lineages must have occurred at least 800 kya [13], much earlier than previously thought.

The 2010 sequencing of the Neanderthal genome indicated that Neanderthals interbred with anatomically modern humans from 80 to 45 kya (at the approximate time that modern humans migrated out from Africa, but before they dispersed into Europe, Asia and elsewhere). Nearly all modern non-African humans have 1% to 4% of their DNA derived from Neanderthal DNA, and in total around 20% of Neanderthal DNA survives in modern humans. It has been generally thought that competition from Homo sapiens probably contributed to Neanderthal extinction around 30,000 years ago (although there is some evidence suggesting a possible survival until around 24,000 years ago). They could have co-existed in Europe for as long as 20,000 years. The recent evidence on interbreeding could indicate that a true “extinction” of Neanderthals may not have occurred, but that they may have been absorbed into H. sapiens [15].

However, while modern humans share some nuclear DNA with the extinct Neanderthals, the two species do not share any mitochondrial DNA, which in primates is always maternally transmitted. This observation has prompted the hypothesis that whereas female humans interbreeding with male Neanderthals were able to generate fertile offspring, the progeny of female Neanderthals who mated with male humans were either rare, absent or sterile.

Neanderthal family

Neanderthals grew to around the same height as modern humans but had stockier andmore muscular bodies. Their brain size was around 1600 cm3 for men and 1300 cm3 for women, within the range of modern humans. They had cold adaptions (body fat storage and enlarged noses) and used a wide range of foods, including hunting megafauna such as mammoth. They were capable of speech, and had various cooking techniques, crafted simple clothes and boats. Some have made claims for evidence of cave art and religious beliefs.

Denisovans

Denisovan woman

In 2008, archaeologists working at the site of Denisova Cave in the Altai Mountains of Siberia uncovered a small bone fragment from the fifth finger of a juvenile member of a population now referred to as Denisova hominins, or simply Denisovans. Artifacts, including a bracelet, excavated in the cave at the same level were carbon dated to around 40,000 years ago. As DNA had survived in the fossil fragment due to the cool climate of the Denisova Cave, both mtDNA and nuclear genomic DNA were sequenced.

The full genomic sequence suggested the Denisovans belonged to the same lineage as Neanderthals, with the two diverging shortly after their line split from that giving rise to modern humans. The date of the split depends on assumptions made about average mutation rates, ranging from 200-744 kya.

Although Denisovan remains have been found at only one Siberian site, DNA analysis

Denny

shows they were widespread. The evidence suggests that Denisovans, Neanderthals and modern humans co-existed for 30 to 40 thousand years, suggesting a much more complex picture of humankind during the Late Pleistocene, with interbreeding occurring between all three groups. The most direct evidence of this is the recent discovery of a 13-year-old girl (nicknamed Denny) who lived in that cave about 90,000 years ago. DNA analysis revealed that her mother was a Neanderthal and her father was a Denisovan [16]. HLA types (genes relating to the immune system) from Denisovans and Neanderthal represent more than half the HLA types of modern Eurasians, indicating strong positive selection for these genes.

Evidence has also been found for as much as 6% of the genomes of some modern Melanesians to derive from Denisovans, indicating limited interbreeding in Southeast Asia, likely with two distinct groups of Denisovans [17]. Denisovans potentially survived as late as 30,000 to 14,500 years ago in New Guinea, where they interbred with ancestors of modern Homo sapiens. Modern Papuans carry hundreds of gene variants from two deeply divergent Denisovan lineages that separated over 350 kya.

Other recently extinct human species

Floresiensis

Homo floriensis

Homo floresiensis nicknamed “hobbit”) is a pygmy archaic human which inhabited the island of Flores, Indonesia, until it went extinct about 12,000 years ago after the arrival of modern humans. Fossils were first discovered in 2003 and it is thought to have been a descendant of a population of Homo erectus that reached the island around 1 million years ago from mainland Asia, or the descendants of an unknown species of human that might’ve migrated out of Africa even earlier. With its extinction, Homo sapiens remained the only human species left on Earth.

Luzonensis

Homo luzonensis

Just last year, fossils discovered in Northern Luzon in the Philippines in 2010 were identified as a new species of human, Homo ludonensis, possibly another pygmy species [18]. Their ancestors, who may have been Homo erectus, arrived in Luzon around 630-770 kya, and would have needed to make a substantial sea crossing to reach it. The remains found in 2010 are at least 50,000 years old.

Naledi

Homo naledi

In 2013, hundreds of fossils were found in a South African cave of a previously unknown species named Homo naledi. They had an unusual mix of archaic and modern anatomy: arms adapted for climbing and feet adapted for walking. They had a small brain around 460-610 cm3, weighed about 40-55 kg and were around 1.46m tall. The fossils were dated to around 300 kya (236-335 kya). It is thought that H. naledi branched off from from  an early Homo species, probably H. erectus, at least 900 kya. H. naledi were probably hunter-gathers eating a mixed diet, and likely used stone tools.

Homo sapiens

Homo sapiens

Homo sapiens evolved in Africa between 350 and 260 kya [19] with an expansion in skull and brain size and elaboration of stone tool technologies. The most recent evidence suggests that Homo sapiens did not evolve from Heidelberg Man in Africa as previously thought but either directly from Homo erectus or Homo ergaster, or from an as yet undiscovered descendant species. Anatomically modern humans evolved from archaic Homo sapiens in the Middle Paleolithic, about 200 kya years ago, only around 20,000 years before Mitochondrial Eve lived.

Homo sapiens migrated out of Africa between 70,000 to 50,000 years ago, and along the way, interbred with the Neanderthals, the Denisovans, and perhaps with many other as yet unknown human species both outside and within Africa based on DNA evidence.

There is much less variation in the DNA of modern humans than is usual for most species. This may have resulted from their relatively recent evolution or the possibility of a population bottleneck resulting from cataclysmic natural events such as the Toba volcanic catastrophe 75 kya. Distinctive genetic characteristics have arisen, however, primarily as the result of small groups of people moving into new environmental circumstances. These adapted traits are a very small component of the Homo sapiens genome, but include various characteristics such as skin color and nose form, in addition to internal characteristics such as the ability to breathe more efficiently at high altitudes.

As I already mentioned, the modern H. sapiens who left Africa around 70 kya met and mated with Neanderthals and Denisovans and with at least one other unknown species of archaic human. As Andrew Sorensen has described human evolution is appearing more and more like a “braided stream”, rather than a classical tree of evolution [20]. Our understanding of human evolution is evolving rapidly with new discoveries and genetic analyses, so no doubt this summary will almost certainly need revision as more information becomes available. Perhaps we should cease to think of Neanderthals and Denisovans as extinct species outcompeted by upstart Homo sapiens out of Africa, and start to think of H. erectus, archaic H. sapiens, Neanderthals and Denisovans all as our ancestors.

We have now reached the point where Mitochondrial Eve lived 178,000 years ago in Southern Africa and join the story of my maternal ancestors set out in a previous post. I will turn in a future post to the story of Y-Chromosome Adam, who likely lived in West Africa around 275,000 years ago, and trace my ancestral migration path back to him.

Our family album. I include Heidelberg man as the direct ancestor of our Neanderthal and Denisovan ancestors

References

[1] Stringer, C. What makes a modern human. Nature. 2012; 485 (7396): 33–35. doi:10.1038/485033a

[2] Based on an average generation length of 25 years. This is approximately correct for hunter-gatherer cultures of us sapiens, and also for chimpanzees and gorillas. Average generation length is shorter for gibbons at around 9 years, and its likely that average generation length for hominids up to the split between hominins and humans was ranged from around 9 years 20Mya upwards to 20-25 around the time of Australopithecus.

[3] Sawyer GJ, Deak V. The Last Human: A guide to twenty-two species of extinct humans. NY: Yale University Press, 2007.

[4] Barbara J King. When did human speech evolve? National Public Radio, 5 Sept 2013. https://www.npr.org/sections/13.7/2013/09/05/219236801/when-did-human-speech-evolve

[5] Michael Balter. Human language may have evolved to help our ancestors make tools. Science DOI: 10.1126/science.aaa6332, 13 Jan, 2015. https://www.sciencemag.org/news/2015/01/human-language-may-have-evolved-help-our-ancestors-make-too

[6} Rogers AR, Harris NS, Achenbach AA.Neanderthal-Denisovan ancestors interbred with a distantly related hominin. Science Advances. 2020 Feb 20 : eaay5483 https://advances.sciencemag.org/content/6/8/eaay5483

[7] Scharping N. An ancient tooth is revealing more about our human ancestors. Discover Magazine, 1 April 2020. https://www.discovermagazine.com/planet-earth/an-ancient-tooth-is-revealing-more-about-our-human-ancestors

[8] Welker, F., Ramos-Madrigal, J., Gutenbrunner, P. et al. The dental proteome of Homo antecessor. Nature (2020). https://doi.org/10.1038/s41586-020-2153-8

[9] De Boer B. Loss of air sacs improved hominin speech abilities. J Human Evolution. 2012;62 (1): 1–6. doi:10.1016/j.jhevol.2011.07.007.

[10] Lozano M, Mosquera M, De Castro J-M, Arsuaga J-L, Carbonell E. Right handedness of Homo heidelbergensis from Sima de los Huesos (Atapuerca, Spain) 500,000 years ago. Evolution and Human Behavior. 2009:30 (5): 369–376. doi:10.1016/j.evolhumbehav.2009.03.001.

[11] Martínez I, Rosa M, Quam R, Jarabo P, Lorenzo C, Bonmatí A, Gómez-Olivencia A, Gracia A, Arsuaga JL. Communicative capacities in Middle Pleistocene humans from the Sierra de Atapuerca in Spain. Quaternary International. 2013;295: 94–101. doi:10.1016/j.quaint.2012.07.001

[12] Stringer, C. (2012). The status of Homo heidelbergensis (Schoetensack 1908). Evolutionary Anthropology. 21 (3): 101–107. doi:10.1002/evan.21311

[13] Gómez-Robles, A. (2019). Dental evolutionary rates and its implications for the Neanderthal–modern human divergence. Science Advances. 5 (5): eaaw1268. doi:10.1126/sciadv.aaw1268.

[14] Prüfer K, Racimo F, Patterson N, et al. The complete genome sequence of a Neanderthal from the Altai Mountains. Nature 2014;505, 43–49. https://doi.org/10.1038/nature12886

[15] Ko KH. Hominin interbreeding and the evolution of human variation. J Biol Res (Thessalon). 2016 Jul 16;23:17. doi: 10.1186/s40709-016-0054-7.

[16] Warren M. Mum’s a Neanderthal, Dad’s a Denisovan: First discovery of an ancient-human hybrid – Genetic analysis uncovers a direct descendant of two different groups of early humans. Nature. 2018;560 (7719): 417–418. doi:10.1038/d41586-018-06004-0

[17] Jacobs GS, Hudjashov G, Saag L, et al. Multiple Deeply Divergent Denisovan Ancestries in Papuans. Cell. 2019 May 2;177(4):1010-1021.e32. doi: 10.1016/j.cell.2019.02.035.

[18] Détroit F, Mijares AS, Corny J. et al. A new species of Homo from the Late Pleistocene of the Philippines. Nature. 2019; 568, 181–186. https://doi.org/10.1038/s41586-019-1067-9

[19] Schlebusch CM, et al., Southern African ancient genomes estimate modern human divergence to 350,000 to 260,000 years ago. Science 2017;358(6363):652-655. https://science.sciencemag.org/content/358/6363/652.full

[20] Little B. How did Humans Evolve. History Stories. 5 Mar 2020. https://www.history.com/news/humans-evolution-neanderthals-denisovans. Accessed 28 March 2020.

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