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Homo heidelbergensis

Extinct species of archaic human

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Mandible (lower jaw) of the type specimen of Homo heidelbergenis from Mauer near Heidelberg, Germany (Original)
Gerbil · CC BY 3.0

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Through Homo

H. heidelbergensis was described by German anthropologist Otto Schoetensack in 1908 based on a jawbone, Mauer 1, from a sand pit near the village of Mauer — 10 km (6.2 mi) southeast of Heidelberg. It was the oldest identified human fossil in Europe, and Schoetensack described it as an ancestor race of living Europeans. By the mid-20th century, all archaic human taxa were lumped as subspecies of either H. erectus or H. sapiens, with the former evolving into the latter without any coexistence. The species was usually lumped as H. e. heidelbergensis. While its utility was complicated by its definition on a jawbone (which is rarely ever found, and otherwise bears few diagnostic features) British physical anthropologist Chris Stringer revived the species in 1983, redefining it as a Euro-African ancestor of modern humans and Neanderthals using namely Kabwe 1, Petralona 1, Bodo, and Arago. These skulls are united mainly by their supraorbital torus (brow ridge) anatomy.

H. heidelbergensis may have evolved from H. ergaster (African H. erectus) possibly following an intense population bottleneck 800,000 to 900,000 years ago. Populations dispersed into Europe by 700,000 years ago, spreading Late Acheulean stone tools, and settlements became more permanent by 500,000 years ago. H. heidelbergensis may have been an active hunter of big game, including straight-tusked elephant and Stephanorhinus rhinoceroses, but at least some populations also subsisted significantly on foodplants and small game. Fire does not seem to have been used frequently, but huts and temporary shelters may have been constructed at least around Europe. There are some instances of nondescript etchings on pebbles, as well as modified and heated ochre, which could have been done with symbolic intentions.

Research history

Classification

Raciology

On 21 October 1907, miners recovered a large human mandible (lower jaw) about 24.1 m (79 ft) down the Grafenrain sand pit near the village of Mauer — 10 km (6.2 mi) southeast of Heidelberg. German geologists Ernst Wilhelm Benecke and Adolf Sauer had earlier characterised the site as diluvial deposits dating to the Tertiary. Mauer 1 was the oldest European human fossil at the time. German anthropologist Otto Schoetensack made the first report of the skull in 1908, classifying it as a new human species, Homo heidelbergensis. After Neanderthals (H. neanderthalensis), it was the second-named fossil species in the genus Homo.

Schoetensack noted the jaw's exceptionally primitive anatomy with its massive size and lack of chin, yet more modern traits such as small teeth. Therefore, he concluded Mauer 1 must represent an ancient European ancestor, which he claimed was further supported by several ontogenetic developments in Europeans. He also claimed that the many similarities with non-human apes indicate that Mauer 1 lies near the last common ancestor of apes and humans.

Allerdings sind die Maße groβ [in Mauer 1], wenn man moderne europäische Objekte zum Vergleich heranzieht. Sowie man aber diesen auf jetzige niedere Rassen ausdehnt, verschwindet die Differenz.

The dimensions are large [in Mauer 1] when modern European objects are used for comparison. However, as soon as one extends this to present-day lower races, the difference disappears.

— Otto Schoetensack, 1908

In 1909, Croatian archaeologist Dragutin Gorjanović-Kramberger recommended renaming Mauer 1 "H. amentalis" ("chinless") as, at that time, every other jaw classified in the genus Homo had at least a weak chin. In 1909, Italian palaeontologist Guido Bonarelli proposed erecting a new genus as "Palaeanthropus heidelbergensis" to recognise its age and primitiveness. German anthropologist Hans Weinert suggested "Europanthropus heidelbergensis", and German anthropologist Ludwig Wilser "Rhenanthropus heidelbergensis" for a similar reason. In 1927, Czech-American anthropologist Aleš Hrdlička considered it a European variant of the Java Man "Pithecanthropus erectus" (now Homo erectus erectus). In 1928, German anatomist Franz Weidenreich made a similar opinion. In 1937, American archaeologist Theodore D. McCown and British anatomist Sir Arthur Keith suggested extending "Palaeoanthropus" to any other fossil with many Neanderthal features, with "P. heidelbergensis" as the oldest member. They also included "P. neanderthalensis" (specifically La Chappelle-aux-Saints 1 and Neanderthal 1), "P. ehringsdorfiensis", "P. krapinensis", and "P. palestinensis".

Modern evolutionary synthesis

By the middle of the century with the formulation of modern evolutionary synthesis, the common convention was to relegate all ancient human specimens into the genus Homo, and designate only a single species of Homo at any point in time: H. erectus which evolved into H. sapiens (anagenesis). The many defined species of archaic humans, including heidelbergensis, were generally lumped as subspecies of either H. erectus or H. sapiens. This left H. erectus and H. sapiens considerably polytypic and anatomically variable. In 1963, Russian-American geneticist Theodosius Dobzhansky noted that Middle Pleistocene Western and Eastern Eurasia — the former represented by H. e. heidelbergensis and the Tunisian H. e. mauritanicus; and the latter by the Chinese H. e. pekinensis and H. e. erectus — had significant anatomical and technological differences (Movius Line). He forwarded the possibility that these two groups represent distinct but contemporary species (cladogenesis), with H. erectus in the East, and the ancestors of H. sapiens in the West.

In 1972, American palaeoanthropologist Bernard Campbell divided Middle Pleistocene H. erectus into two chrono-subspecies (that is, designating a given subspecies as more closely related to certain subspecies over others); one group including H. e. heidelbergensis, H. e. mauritanicus, and H. e. pekinensis; and the other the Tanzanian H. e. leakeyi (Olduvai Gorge Bed IV) and H. e. erectus. At this point, H. e. heidelbergensis included at least Mauer 1 and the Hungarian Vértesszőlős 2.

Cladistics

In 1974, British physical anthropologist Chris Stringer noted that the Greek Petralona 1 was anatomically more comparable to the Zambian Kabwe 1, Mauer 1, and Vértesszőlős 2 than to East Asian Middle Pleistocene H. erectus. He proposed classifying them as H. s. heidelbergensis — a widespread Euro-African clade, and the last common ancestor of modern humans (H. sapiens sapiens) and Neanderthals (H. sapiens neanderthalensis). At the time, he was hesitant to revive entire species for fear of recluttering human taxonomy, but in 1983, he proposed classifying them as a unique species as either H. heidelbergensis or H. rhodesiensis (named in 1921 with Kabwe 1), depending on the inclusion of Mauer 1, as the common ancestor of H. sapiens and H. neanderthalensis. The utility of H. heidelbergensis is complicated by its definition on a jaw, which is an uncommon find in Middle Pleistocene deposits, and additionally has few diagnostic traits. Nonetheless, Kabwe 1, Petralona 1, the Ethiopian Bodo cranium, and the French Arago have normally been discussed altogether as representatives of H. heidelbergensis, united most evidently by their brow ridge anatomy.

Though H. heidelbergensis became a popular designation, in 2000, American anthropologists Sally McBrearty and Alison S. Brooks argued that H. heidelbergensis should be reserved for only the direct ancestors of Neanderthals in Europe. They recommended reviving H. rhodesiensis to house African Middle Pleistocene fossils they believed were directly ancestral to modern humans. Spanish palaeoanthropologist Juan Luis Arsuaga and colleagues made a similar opinion while studying the Spanish Sima de los Huesos hominins — which comprise the vast majority of the Middle Pleistocene human fossil record. They opted to classify every Middle Pleistocene European fossil as a Neanderthal ancestor under the name H. heidelbergensis, and placed the 1 million year old Spanish H. antecessor as the last common ancestor of modern humans and Neanderthals. Stringer disagreed with the inclusion of the Sima de los Huesos hominins in H. heidelbergensis, preferring to classify them as Neanderthals.

In 2010, American palaeoanthropologists Jeffrey H. Schwartz and Ian Tattersall noted that, while the Euro-African H. heidelbergensis has a wide range of anatomical variation, these specimens may be too derived (have too many apomorphies, or unique traits) to represent modern human ancestors — though they could still be closely allied with Neanderthals. In 2011, French anthropologist Aurélien Mounier and colleagues instead extended H. heidelbergensis to encompass Middle Pleistocene specimens all across the Old World, including the Chinese Dali Man and Jinniushan — characterising H. heidelbergensis as an extremely polytypic species and the last common ancestor of modern humans and Neanderthals. They used the 400,000 year old Italian Ceprano Man skull as the "counterpart" of the Mauer 1 mandible to better diagnose the species.

In 2011, Arsuaga and colleagues failed to identify distinctly Neanderthal traits in Mauer 1 — unlike in the Sima de los Huesos hominins and some other Middle Pleistocene Europeans. They recognised two distinct groups occupying Middle Pleistocene Europe: one that was evolving into Neanderthals (pre-Neanderthals), and one that was not (maybe best designated as H. heidelbergensis). In Africa, Stringer noted that some of the specimens he assigned to H. heidelbergensis have similarities with modern humans in the face (such as the Tanzanian Ndutu cranium) while others do not (Kabwe 1 or Bodo).

Multiregionalism

In 2016, Stringer characterised H. heidelbergensis as either the group encompassing the last common ancestor of modern humans and Neanderthals, or a unique branch (more closely related to Neanderthals) which eventually became extinct. Either way, H. heidelbergensis would have lived at the same time as more derived morphs. These interconnected derived populations (not H. heidelbergensis) — dispersed across respectively Africa and Europe — seem to have been slowly accruing apomorphies at different rates, which would eventually culminate in respectively anatomically modern humans and classic Neanderthals (multiregionalism). Congruently, in 2020, the Kabwe 1 skull was dated to roughly 300,000 years ago, living at the same time as the earliest recognised modern human fossils at the Moroccan Jebel Irhoud site. Late-surviving H. heidelbergensis populations may have interbred with modern humans.

While some East Asian Middle Pleistocene fossils have some anatomical similarities to material typically classified as H. heidelbergensis, genetic sequencing of human fossils in the 2010s identified an enigmatic group of archaic humans called the Denisovans — closely related to Neanderthals — dispersed across East Asia. This opened the possibility that these East Asian specimens belong to a different, unique species (one that was also interbreeding with modern humans, as well as Neanderthals), such as H. longi.

The species-level classification of Middle Pleistocene humans is still a contentious matter, popularly termed "the muddle in the middle". H. rhodesiensis is usually considered to be a junior synonym of H. heidelbergensis. In 2022, Serbian–Canadian palaeoanthropologist Mirjana Roksandic instead suggested expanding the definition of H. neanderthalensis to include Middle Pleistocene European specimens with Neanderthal traits, and housing African and non-Neanderthal European specimens in H. bodoensis instead of H. rhodesiensis to avoid honouring Cecil Rhodes. She considered H. heidelbergensis too poorly defined to continue using. Her recommendations have been criticised for oversimplifying the archaeological record, and for violating the principle of priority.

In a 2024 interview with Cell Biology Magazine, Stringer expressed doubts with his earlier conceptions of H. heidelbergensis.

With Homo heidelbergensis, I had the view for a long time that it was the common ancestor of us and Neanderthals about 500,000 years ago. Now I think that's unlikely and that the common ancestor lived further back and did not look like heidelbergensis. That's partly a product of my being around for so long, as well as seeing so much new data and so many changes in thinking.

— Chris Stringer, 2024

Evolution

H. heidelbergensis is thought to have descended from African H. erectus — sometimes classified as H. ergaster. The exact derivation from an ancestor species is obfuscated by a long gap in the human fossil record near the end of the Early Pleistocene. In 2016, Italian anthropologist Antonio Profico and colleagues suggested that 875,000 year old skull material from the Gombore II site of the Melka Kunture Formation, Ethiopia, represents a transitional morph between H. ergaster and H. heidelbergensis, and thus postulated that H. heidelbergensis originated in Africa. A 2023 genetic study of 3,000 people found that the global population was reduced to less than 1,300 individuals between 800,000 and 900,000 years ago. This extreme population bottleneck could have caused the divergence of H. heidelbergensis.

Human dispersal beyond 45°N seems to have been quite limited during the Lower Palaeolithic, with evidence of short-lived dispersals northward beginning after a million years ago. More permanent populations seem to have become established above this parallel about 700,000 years ago. This coincides with the spread of hand axe technology across Europe, possibly associated with the dispersal of H. heidelbergensis and behavioural shifts to cope with the cold climate. Such occupation becomes much more frequent after 500,000 years ago.

According to genetic analysis, the last common ancestor of modern humans and Neanderthal split into a modern human line, and a Neanderthal/Denisovan line, and the latter later split into Neanderthal and Denisovans. According to nuclear DNA analysis, the 430,000 year old Sima de los Huesos hominins are more closely related to Neanderthals than Denisovans — meaning that the Neanderthal/Denisovan, and thus the modern human/Neanderthal split, had already occurred. This suggests that the modern human/Neanderthal last common ancestor had existed long before many specimens typically assigned to H. heidelbergensis did. H. heidelbergensis also seems to have lived alongside modern humans, demonstrated by the 300,000 year old Kabwe 1, which could further cast doubt on its position as a modern human ancestor.

A 2021 phylogeny of some Middle Pleistocene fossils using tip dating:

A subsequent version of this analysis found a different topology, with Homo heidelbergensis as a monophyletic group:

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