Homo heidelbergensis

By Kayleigh Andrews

Photograph of the Mauer Mandible (from Harvati, 2007)

What is Homo heidelbergensis?

Homo heidelbergensis is a species of hominin that was first named by Otto Schoetensack (1908). Used here, hominin refers to the extinct species that are closely related to modern humans, as well as modern humans themselves (Wood and Harrison, 2011). In 1907, a fossilized mandible was found in the Mauer Sands near Heidelberg, Germany. At the time it was discovered, this mandible was one of the most ancient hominin fossils to be found in Europe (Harvati, 2007). Its differences from previously discovered specimens is what led it to be classified as a new species, Homo heidelbergensis, such as a lack of a chin and general thickness (Mounier et al., 2009). Since then, the number of fossils discovered that have been attributed to this species has increased, allowing for more knowledge about this taxon to be established. H. heidelbergensis lived in the Middle Pleistocene, or around 780-127 thousand years ago (kya). This time was characterized by increased hominin migration out of Africa and into Europe, as well as Asia (Harvati, 2009). It is likely that H. heidelbergensis existed alongside other hominin species, such as Homo erectus and Homo neanderthalensis (Dennell et al., 2011), but its exact evolutionary relationships are unclear, and its geographic range has also been the subject of debate. This article will cover this species’ features, evolutionary relationships, important fossils, behaviors, controversies, and more.

Features of the Species

Homo heidelbergensis has several physical features that make it distinct from other species in the genus Homo. When discussing specimens of a species, these physical features are often referred to as morphologies. Some of the morphologies that characterize H. heidelbergensis are (Stringer, 1984; Harvati, 2007; Mounier et al., 2009; Stringer, 2012; Rightmire, 2013):

Strong and continuous supraorbital torus that is depressed at the glabella
Reduced facial prognathism
Broad face
Increased midfacial projection (midline nasal prominence)
Large cranial capacity (average of 1230 cm3)
Relatively long and low cranium
High cranial thickness
Strongly angled occipital bone
Strong and continuous occipital torus
Parallel-sided cranial vault
Increased convexity of the frontal and parietal squama
Greater arching of the temporal squama
Gracile tympanic portion of the temporal bone
Decreased total buttressing of the cranium
Extreme pneumatization
Thick mandibular body
Wide mandibular ramus breadth
Absence of chin

Description of Features

These morphologies are regarded as a combination of characteristics seen in other species of Homo. The features that are similar to species that are likely ancestral to Homo heidelbergensis, such as Homo erectus, are considered more primitive, while other features are more similar to later species such as Homo sapiens and Homo neanderthalensis (commonly called Neanderthals). The primitive features include a strong supraorbital torus (also called a brow ridge), a broad face, a long and low cranium, high cranial thickness, a strongly angled occipital bone, the presence of an occipital torus, wide mandibular rami, and the lack of a chin. This brow ridge was much larger than what is seen in modern humans, with a depression dividing it into right and left halves. The general shape of the face is also broader than that of modern humans. The cranial vault (the area that holds the brain) has many features that resemble H. erectus, including its general shape, the thickness of the bone, and the presence of a strong ridge on the back of the skull for muscle attachment. The features of the mandible are also more primitive and relatively thick (Rosas and Bermúdez de Castro, 1998; Harvati, 2007). Moving on to the features that are more like later Homo species, these are reduced prognathism, increased midfacial projection, a larger cranial capacity, the shape of the sides and front of the cranial vault (the frontal, parietal, and temporal bones), and pneumatization of the skull. Compared to more primitive species, the lower face is less projecting, with a more pronounced area above the nostrils. Many of the features of the shape of the skull relate to an increased cranial capacity, such as greater bulging of the front and sides of the braincase, and greater height (as shown by upwards arching of the temporal bones on the sides of the skull) (Rightmire, 2004). Some other features of the skull that are less primitive, but do not relate to brain size, are a more delicate tympanic portion (where the opening of the ear is located) and reduced buttressing (less prominent muscle attachment locations). A somewhat unique feature, beyond the massive brow ridge, is extreme pneumatization (increase in size of air spaces within the bone), theorized to relate to reducing the weight of a larger skull (Stringer, 1984; Seidler et al. 1997; Harvati, 2007; Stringer, 2012).

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Homo heidelbergensis has many primitive traits, but they often do not present exactly the same as seen in ancestral species. One of the most notable traits of this species is the presence of a strong and continuous supraorbital torus, which is a bony ridge present over the eyes, often called the brow ridge. While this morphology is also seen in H. erectus, including the depression of the ridge seen at the glabella (point on the brow ridge between the eyes), it is more pronounced and laterally flaring in H. heidelbergensis. The occipital bone (which forms the rear of the braincase) is angled, meaning that there is a distinct difference in angle between the upper and lower parts of this bone. A strong occipital torus (another bony protrusion) is situated at this angle, again similar to H. erectus. This torus is indicative of the presence of heavy nuchal musculature, as it is the attachment site for the muscles that extend down the back of the neck. The general broadness of the face, suggested by a wide interorbital breadth (space between the eyes) and more laterally flaring zygomatics (the cheekbones), is also considered to be more primitive. Overall, the shape of the braincase is long and low, showing more resemblance to species with lower cranial capacity. The overall thickness of the bone (of both the cranium and the mandible) is also more indicative of ancestral species, along with both the lack of a chin and wide mandibular rami (the vertical parts of the mandible that articulate with the rest of the skull) (Rosas and Bermúdez de Castro, 1998; Harvati, 2007; Stringer, 2012).

Because of these more ancestral features that Homo heidelbergensis displays, many fossils that were later attributed to H. heidelbergensis were originally classified as H. erectus. However, many of the characteristics of H. heidelbergensis show more similarities to later species, such as Homo sapiens (the same species as modern humans) and Homo neanderthalensis (commonly called Neanderthals). For example, the gracile tympanic portion of the temporal (the area where the external opening of the ear is located) is like those seen in these species, with a less robust appearance. Many of the more derived morphologies of the braincase relate to increased cranial capacity. For example, the increased convexity of the flattened portions of the parietal and frontal bones (found on the upper middle portion of the braincase and the front of the braincase, respectively) and the curving seen in the temporal squama (flat portions of the temporal bones, located inferior to the parietal bones on the sides of the cranium) (Rightmire, 2004). The vault is also parallel-sided, meaning that the sides of the vault are vertical rather than angled inwards toward the top of the skull. The cranial capacity (the volume of the inside of the skull, which is related to brain size and therefore intelligence) seen in H. heidelbergensis is greater than 1100 cm3, with an average of 1230 cm3 (Rightmire, 2013). This capacity overlaps with those seen in Homo erectus, Homo sapiens, and Neanderthals, but on average is significantly larger than the volume seen in H. erectus (Rightmire, 2004). The total facial prognathism (forward projection of the lower face) is reduced from what is seen in H. erectus, and there is some midfacial projection (measured by nasal prominence, the area above the nasal opening) which is seen in H. sapiens and Neanderthals. Decreased total buttressing is also seen in the skull, which means there are less defined muscle attachment sites than displayed by previous species. There are also some features H. heidelbergensis has that are not seen in any other Homo species, in addition to the massive supraorbital torus. Various parts of the skull are extremely pneumatized, as revealed by CT-scanning of different fossils, meaning that the sinuses (spaces that contain air) within the bones are large and extensive, especially in the lateral parts of the supraorbital torus into the frontal bone. It is theorized that this pneumatization relates to the ability to have a lighter, larger skull (Stringer, 1984; Seidler et al. 1997; Harvati, 2007; Stringer, 2012). Overall, this unique combination of traits is why Homo heidelbergensis is regarded by many as a distinct species.

Behaviors

The sites in which specimens of H. heidelbergensis have been found indicate the possible behaviors of this species. These include the increased use of rock shelters and caves, more sophisticated tool technologies, hunting, and the utilization of fire. The tools found at these sites, such as bifacial Acheulean hand-axes, display greater standardization and specialization than the Oldowan tools made by earlier hominins (Rightmire, 2004). It is likely that these tools were used for multiple purposes, both in the processing of food, as well as the creation of other tools such as wooden spears. These spears were used for hunting (Wagner et al., 2011), and they may have used stone points as spear tips (Wilkins et al., 2012). Archaeological evidence indicates that this species often used fire to cook this meat at this sites, shown by the presence of burned animal bones. This means that they had some level of control over the creation of fire. These behaviors indicate increased intelligence and moving towards greater social organization as seen in later species (Harvati, 2007; Kuhn and Stiner, 2019).

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During the period in which this species lived non-open-air sites became more common (e.g. the use of rock shelters and caves), which may have been partially due to the overall lower temperatures seen in the Middle Pleistocene (Dennell et al., 2011). These locations often show artifacts assemblages accumulated over long periods of time, suggesting a tendency to reuse sites. In other words, it indicates established base camps that groups tended to return to, which is similar to behavior seen in modern hunter-gatherers. The tools found at these sites show technological diversity, from large bifacial hand-axes to more delicate blade tools. While this species made tools from multiple industries, many of the tools seen are characteristic of the later Acheulean industry, such as bifacial hand-axes, which are more advanced than the first Oldowan tools created by hominins (Rightmire, 2004). Tools like these were often used in the processing of meat, and it is likely that staple meat sources were obtained through hunting, rather than scavenging. Wooden spears have been found at some sites, indicating that this may have been a way that H. heidelbergensis hunted (Wagner et al., 2011), and there is also evidence that stone points were used as spear tips (Wilkins et al., 2012). Many of the cave sites also show evidence of the frequent use of fire. While it is not known exactly when the regular harnessing of fire by hominins began, it seems that H. heidelbergensis could use it reliably by the mid-Middle Pleistocene (around 450 kya). Often, hunted animals were processed at the kill site, and better pieces of meat were transported to caves in order to cook them, indicated by the presence of burned bones. This transportation of meat away from the kill site to places used as home bases is also evidence of food sharing, a behavior also seen in modern humans (Kuhn and Stiner, 2019). It has been theorized that these advances in lithic technology as well as social organization are connected to the increase in brain size and encephalization (increase in cranial capacity) seen in H. heidelbergensis. The more sophisticated use of tools, hunting techniques, and other behaviors described here require increased intelligence as well as increased social cooperation (Rightmire, 2004; Harvati, 2007). Sites like Sima de los Huesos in Spain contain hominin fossils that show striations on the anterior teeth, made by stone tools, indicating the use of these implements alongside an individual’s teeth in food processing activities. The orientation and shape of these striations can indicate handedness, also called hand laterality. Research has revealed that H. heidelbergensis was preferentially right-handed, similar to modern human populations. The presence of handedness also indicates brain laterality, which is a result of evolution of the brain further towards the brain structure of modern humans, and is related to things like language-capability and more sophisticated tool use (Lozano et al., 2009).

Place in Hominin Evolution

Figure 1. A tree illustrating the evolution and geographic distribution of Homo in the Pleistocene (from Rightmire, 1998).

Time and Geographic Range

Of the remains that have been attributed to Homo heidelbergensis, the majority date to the Middle Pleistocene, ranging from 780 to 127 kya. During this time, hominins were already widespread across Africa. The beginning of this period marks the first evidence of hominins in Europe, implying that this is when wider human colonization into Europe first began (Harvati, 2009). While it was relatively cooler than the world today, the Mid Pleistocene was still interspersed with warmer intervals. It is likely that the most migration occurred during these interglacial periods due to less severe conditions (Dennell et al., 2011). Fossils of H. heidelbergensis have been found across a wide geographic area, including southern and eastern Africa, and ranging from western to central Europe (Rightmire, 2013). The comparison of the specific fossils that will be discussed later (Mauer 1, Petralona 1, and Kabwe 1) is part of what led to the development of a Eurafrican concept of the species. Fossils of a similar age with similar morphologies have also been found in Asia, but it is debated whether they should be included in the species (Rightmire, 1998; Stringer, 2012).

Phylogeny

The placement of Homo heidelbergensis within the hominin evolutionary tree is unclear. As previously established, various morphologies that characterize the species are also observed in multiple other species of Homo that came both before and after H. heidelbergensis. However, the exact evolutionary path is unknown. Some general possibilities, as demonstrated by the figure above, are that H. heidelbergensis is a stem group that split from H. erectus and later speciated into H. sapiens and Neanderthals (Fig. 2a and 2b). One theory (Fig. 2a) considers H. sapiens and Neanderthals to be distinct species, which implies that the features seen in H. heidelbergensis are enough to classify it as a distinct species as well (or H. rhodesiensis, if the Mauer mandible is not grouped with other specimens of this species). An alternative theory (Fig. 2b) considers a similar path, however with H. heidelbergensis and H. neanderthalensis instead considered to be subspecies of H. sapiens, with H. sapiens heidelbergensis representing a more archaic form of H. sapiens. However, this grade classification as archaic vs. modern has generally fallen out of use over time (Harvati, 2009). Another possibility (Fig. 2c) shows that the exact phylogenetic position of H. heidelbergensis cannot be determined but is close to the divergence of Neanderthals from modern H. sapiens (Stringer, 1983). A popular evolutionary theory for those researchers that recognize H. heidelbergensis as a distinct species is that H. heidelbergensis represents a daughter lineage of Homo erectus that split at or before the beginning of the Middle Pleistocene and spread throughout Africa and Europe. After this, Neanderthals arose from the H. heidelbergensis population in Europe, while H. sapiens speciated from H. heidelbergensis in Africa, making it the last common ancestor of these two species (Rightmire, 2001; Harvati, 2007; Stringer, 2012). The exact evolutionary relationships of H. heidelbergensis are still the subject of much debate and controversy, and will be further discussed in a later section.

Figure 2. Illustration of the possible phylogenetic relationships of the Petralona and Broken Hill fossils; a) Shows H. heidelbergensis or H. rhodesiensis (if Mauer mandible is not included) as a distinct species that split off from H. erectus and later evolved into H. neanderthalensis and H. sapiens, b) View that considers the same path as scenario (a), but instead considers H. heidelbergensis or H. rhodesiensis as an archaic form of H. sapiens, c) Scenario that instead considers the exact evolutionary position of H. heidelbergensis to be unknown, but is likely descended from H. erectus and somewhere around the divergence into H. neanderthalensis and H. sapiens (from Stringer, 2012).

Important Fossils

In the study of the evolutionary relationships of past species, the naming of fossils reflects where they were discovered and the number of remains found at the same site. One fossil, often the first of the species discovered, is referred to as the holotype or type specimen. This specimen is used as the standard for determining the morphologies of the species. There are numerous H. heidelbergensis fossils that could be included here, but only three will be focused on to give a more concise overview and due to their prominence in literature concerning the species.

Figure 3. Superior and lateral views of the Mauer mandible; arrows indicate main morphological features in the definition of H. heidelbergensis (from Mounier et al., 2009).

Mauer 1

This fossil is considered to be the type specimen of Homo heidelbergensis. As previously described, it was the first remains found that were attributed to this species. In 1907, workers recovered a mandible from a sandpit near the village of Mauer referred to as the “Mauer sands”, southeast of Heidelburg, Germany (Schoetensack, 1908; MacCurdy, 1909).

Often referred to as the “Mauer mandible”, it is one of the more significant early discoveries of hominin remains in Europe. Some of the notable features of this fossil are the lack of a chin, the general thickness of the mandibular body, and the breadth of the mandibular ramus (the vertical portion), which differ from other hominin species that lived at the same time and lent to its classification as a new species. Well-developed features on the lateral surface of the mandibular body also indicate strong muscle attachments. Interestingly, multiple mental foramina (small openings) are present on the mandibular body, which is similar to what is seen in Neanderthals (Mounier et al., 2009). Another notable feature is the relatively small size of the teeth, which are comparable to those of modern humans (MacCurdy, 1909).

This fossil was found alongside mammal remains from the same time period, allowing for its age to be estimated to the early Middle Pleistocene period through the comparison of these mammal fossils to those found in other locations. More absolute dating techniques, such as isotope analysis, were later used to date the mandible to around 640-600 kya (Mounier et al., 2009). The types of animals found in the same layer as the mandible also suggest that there was a warmer climate with milder cold weather during this individual’s lifetime. There is evidence that this individual was an adult at the time of death, shown by the presence of fully erupted dentition and moderate wear present on the crowns of the teeth (MacCurdy, 1909). Interestingly, the direction of this wear on the teeth indicates that this individual may have had a mostly vegetable diet. At the site flint tools have also been discovered, but it is difficult to definitively associate them with the mandible (Wagner et al., 2011).

Petralona 1

Commonly referred to as the “Petralona cranium” in literature, this fossil is a relatively complete skull (minus the mandible). It was discovered in a cave in Katsika Hill near Petralona, Greece in 1960 by local villagers (Harvati, 2009). At the time it was found, the cranium was encased in a pink stalagmitic matrix that had to be manually removed. Unfortunately, this did result in some slight damage to the more delicate aspects of the fossil (Stringer et al., 1979). Interestingly, it was claimed that the fossil was originally found as a full skeleton, but this has been disputed by different accounts. Regardless, if the fossil was originally complete the rest of the bones were lost before any analysis could take place (Poulianos, 1971; Grün, 1996).

Figure 4. Lateral and frontal views of the Petralona cranium (from Rightmire, 1998).

Petralona 1 has a number of classifying physical traits. One of the most prominent is a projecting supraorbital torus, without the presence of a supratoral sulcus (depression between the torus and the frontal bone/forehead). This torus is divided into right and left halves by a depression at the glabella. There is a post-orbital constriction (depression between the facial skeleton and the cranial vault) lateral to the torus. The midline of the face projects more above the nasal openings, and the middle and lower face are relatively flat (does not display prognathism). The frontal bone is also relatively flat above the supraorbital torus. The zygomatic bones (also called cheekbones) are large and flare laterally. The occipital torus presents as an extensive raised area that ends at the junction between the upper and lower parts of the occipital bone. The cranial vault is broadest at the temporal bones, while the parietal bones are comparatively flattened. The temporal squama curve upwards and the tympanic portions of the temporal bones are relatively gracile. The buttressing system (muscle attachment sites) of the cranium is generally reduced (Stringer et al., 1979; Stringer, 1984). It also exhibits a very large nasal opening, and the cranial capacity has been estimated to around 1,230 cm3 (Stringer, 1983; Rightmire, 2004).

The cave in which the cranium was discovered also contained various faunal remains, deposited throughout the cave over a long period of time. These animal remains were previously used for estimating the age of Petralona 1, but since the cranium was found closer to the surface it is likely that these fossils are not directly associated (Stringer, 1983). Despite this, relative dating methods using these animals have placed the individual in the Middle Pleistocene, or around 700 kya. More absolute methods of dating have since been used, and ESR (electron spin resonance) dating of the matrix surrounding the skull has given an age range of 250-150 kya (Grün, 1996). This still places this individual within the Middle Pleistocene, but in the late portion rather than the early. Various tools were also found within the cave, but similar to the faunal remains it is difficult to definitively associate them with the cranium. These implements, mainly chopping tools and awls, were made from both stone and bone. There was also a hearth found at the site, with charred animal bones, indicating that this individual may have known how to harness fire and also cooked meat to consume it (Poulianos, 1971).

Figure 5. Lateral and frontal views of the Broken Hill cranium, scale bar = 1 cm (from Grün et al., 2020).

Kabwe 1

This fossil, referred to colloquially and in literature as “Rhodesian Man” or the “Broken Hill cranium”, was discovered in 1921 by miners looking for lead ore. Instead, they came upon a cavern containing bones and stone artifacts. This happened in Kabwe, Zambia, previously called Broken Hill, Northern Rhodesia (Clark et al., 1947). Arthur Woodward (1921) was the first to analyze and describe this specimen, and he attributed it to a new species that he dubbed Homo rhodesiensis. However, in this article Kabwe 1 will be considered an example of H. heidelbergensis. The controversies surrounding this designation will be further described in a later section.

Kabwe 1 is another relatively complete cranium, with important parts of the skull, such as the braincase and facial skeleton, remaining mostly intact and undistorted. Its most characteristic and remarkable feature is the presence of a very strongly developed and thick supraorbital torus that projects laterally, without the presence of a supratoral sulcus. As seen in other specimens of H. Heidelbergensis, the supraorbital torus is also depressed at the glabella. The face is generally large, as indicated by the wide interorbital breadth and laterally flaring zygomatics. The face is also long, especially below the nasal opening. Any prognathism of the lower face is greatly reduced, with the alveolar region (just above the teeth), and the midface (around the nasal region) showing more protrusion than the rest of the face. The frontal bone is flattened and relatively narrow, allowing for the presence of a post-orbital constriction. The cranial vault is long and low, featuring a sharply angled occipital bone with an occipital torus at this angle. Any buttressing that the cranium displays is relatively reduced. The widest part of the braincase is at the temporal bones, the squama of which curve upwards. These bones also have a gracile tympanic portion. The cranial capacity has been estimated to be around 1280 cm3 (Morant, 1928; Rightmire, 1976; Stringer, 1984; Seidler et al., 1997).

It is reported that at the time of its discovery, this specimen was displaced from its original position, making it difficult to correlate with other hominin remains in the cave. Despite this, a tibia and the midshaft of a femur, were attributed to the same individual as the cranium. The quarrying at the site has also made Kabwe 1 difficult to date. Early attempts placed the fossil generally within the Middle Pleistocene, and the analyzation of different faunal remains since then has produced different possible ages, ranging from 600-300 kya. The use of uranium series (U-series) dating on the cranium itself has given an age of around 300 kya. Interestingly, the other hominin skeletal elements that have been associated with the skull show U-series results of 190-130 kya, meaning these bones may not be from the same individual as the cranium (Grün et al., 2020). Other non-bone items were also found at this site, including various stone tools such as flake-blades, hammerstones, and various points. The tools can be correlated with faunal remains also found in the cave, as they were most likely used for the processing of these animals for food, evidenced by bones that were found with stone implements embedded in them. (Clark et al., 1947).

Comparing Fossils

All of these fossils demonstrate similar physical traits, hence why all of them have been classified as H. heidelbergensis. Petralona 1 and Kabwe 1 both consist of a complete cranium missing a mandible, making it possible to directly analyze the differences and similarities between the two skulls. Besides the apparent difference in age, as well as geographic location, they also display morphological disparities that some researchers consider enough to classify them as different species. Others think the similarities are sufficient to consider them the same species. Petralona 1 has been considered to display a more Neanderthal-like appearance, with features like reduced alveolar prognathism, a higher nasal bridge, a more projecting midface, and an overall shorter face than Kabwe 1. However, these morphologies are still less prominent in Petralona 1 than in Neanderthals. The post-orbital constriction in Petralona 1 is comparatively reduced due to increased pneumatization of its lateral frontal sinuses (Seidler et al., 1997), and its nasal aperture is wider than Kabwe 1 (and more Neanderthal-like). The orientation of the zygomatics (cheekbones) seen in Petralona 1 is also often considered to be a Neanderthal trait, however this same morphology is seen in Kabwe 1. Moving onto the cranial vault, both fossils have a lower and more elongated braincase, with Neanderthals displaying a generally higher braincase than seen in these specimens (Harvati, 2009). Some other features that both specimens have in similar are a more gracile tympanic portion of the temporal, as well as arching of the temporal, similar occipital proportions, and a relatively similar brain size (Rightmire, 2008). Looking at the most remarkable feature of both craniums, the supraorbital torus is relatively massive and has the same general shape with a depression at the glabella (Stringer, 1984), as well as lacking a supratoral sulcus between the torus and the frontal bone (Rightmire, 1976; Stringer et al., 1979). For those that consider these specimens to belong to the same species, these differences can be explained by simple regional variation between different groups, and the similarities are enough to classify them both as Homo heidelbergensis.

Controversies

In the analysis of fossils, it is not unusual for the classification of different species to change over time. It is not uncommon for hominin fossils to be reclassified as more specimens are discovered and new studies are performed. Since Homo heidelbergensis was first named as a species, there have been disagreements over how it should be categorized. While Schoetensack (1908) first classified it as a new, distinct species within the genus Homo, H. heidelbergensis went largely unused as a designation in the early 20th century. At that time, many of the fossils that some researchers now consider to be H. heidelbergensis were lumped in with Homo erectus or Homo neanderthalensis. Since then, the ability to classify fossils as H. heidelbergensis has remained complicated, due to the mosaic of H. erectus and later Homo features that they display (Harvati, 2007; Stringer, 2012).

While the view of classifying different specimens as “archaic” members of H. sapiens has largely been moved past, there is still debate over whether Homo heidelbergensis should be considered its own, distinct species. Some consider European H. Heidelbergensis to instead be early members (also called a chronospecies) of Homo neanderthalensis, meaning that any African fossils would instead be attributed to a different species, Homo rhodesiensis, as classified by Woodward (1921) (Fig. 6a). This deviates from the view of H. Heidelbergensis as a dstinct species that inhabited both Africa and Europe (Fig. 6b). It also places the last common ancestor to H. sapiens and Neanderthals as Homo erectus or even Homo antecessor rather than Homo heidelbergensis (Harvati, 2007; Harvati, 2009). This view encompasses an “accretion” hypothesis, which theorizes that more Neanderthal-like features appeared over time, facilitated by isolation in colder climatic conditions, making H. heidelbergensis instead early members of Homo neanderthalensis (Rightmire, 1998). Accompanying this alternative view of evolutionary relationships, there are also differing views on whether some fossils should be considered H. heidelbergensis or not. For example, Kabwe 1 and other fossils from Africa such as Bodo, and the fossils found at Sima de los Huesos that show a combination of Neanderthal-like and heidelbergensis-like features. The Petralona cranium is also the subject of such scrutiny, and is often compared to Neanderthals (Stringer, 2012). These controversies over classification reflect the differences in opinion about the phylogeny (or lack of) of Homo heidelbergensis. Another controversy to be addressed is the classification of some Middle Pleistocene Asian hominins, particularly in China, that were more advanced than Homo erectus. Similar to specimens of H. heidelbergensis, these fossils display a mix of more primitive and more advanced Homo characteristics and were originally classified as H. erectus. However, they do show morphologies that may point towards a designation as H. heidelbergensis, despite some differences in features (Rightmire, 1998). Since its discovery, Homo heidelbergensis has remained a controversial taxon.

Figure 6. Alternative scenarios for Middle Pleistocene human evolution; a) European Middle Pleistocene specimens are viewed as early members of the Neanderthal lineage and assigned to H. neanderthalensis along with later Neanderthals (accretion hypothesis), with the last common ancestor of Neanderthals and modern humans shown as H. erectus, b) H. heidelbergensis is shown as a cross-continental species comprising European, African and possibly Asian Middle Pleistocene human fossils, and shown as the last common ancestor of Neanderthals and modern humans (from Harvati, 2007).

Figure 7. Alternative evolutionary trees showing the relationships among Homo erectus, Middle Pleistocene hominins, Neanderthals, and modern humans - solid bars depict the duration (first appearance date and last appearance date) estimated for each species, broken lines indicate likely links of ancestors with descendants; a) Shows Mid-Pleistocene hominins and later populations grouped into a series of more archaic to more modern grades in a broad H. sapiens category; B) Shows at least two lineages in addition to H. erectus and H. sapiens, with a European branch of Neanderthals that can be traced back into the Middle Pleistocene, and the claim that the oldest European hominins share characteristics with H. neanderthalensis and can be distinguished from a second lineage that is rooted in Africa (recognized as H. rhodesiensis); C) Perspective that holds that morphological differences among the most ancient European and African specimens are minor and can be attributed to geography and intragroup variation, and if the Mauer mandible is included within such a broadly construed hypodigm then the appropriate name for this species is H. heidelbergensis (from Rightmire, 2013).

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