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Homo heidelbergensis

2014; Elsevier BV; Volume: 24; Issue: 6 Linguagem: Inglês

10.1016/j.cub.2013.12.048

1879-0445

Laura T. Buck, Chris Stringer,

Who is Homo heidelbergensis? H. heidelbergensis is a critical human species in the Middle Pleistocene (∼130–780 thousand years ago (ka)). We know from several beautifully preserved crania that this species had a large brain, within the lower range of modern human variation, and a less robust face than early fossil humans. We know from their long bones that they were tall, strong people. From their associated archaeology we know they were capable of producing beautiful tools such as the large handaxes found in huge numbers at Boxgrove in Sussex. But there are many unanswered questions: who exactly belongs to the species Homo heidelbergensis, where did they live, how do they fit into the human family tree, and are they a separate species at all? What's the problem? The Middle Pleistocene is often referred to as the 'muddle in the middle' — an apt description given the great debate over which hominin species should be recognised and the attribution of fossils to those species. It is generally believed that the lineages of modern humans (Homo sapiens) and Neanderthals (Homo neanderthalensis) originated during this time, but the nature of their last common ancestor, when and where this ancestor lived, and what the ancestor should be called are continuing sources of controversy. One popular designation for the last common ancestor is Homo heidelbergensis. Presumably, it was discovered in Heidelberg..? Yes, the type specimen for H. heidelbergensis (against which other specimens must be compared to judge their inclusion in the species) is a mandible found in 1907 at Mauer, near Heidelberg in Germany. Without the inclusion of this fossil, no group of specimens can formally be called H. heidelbergensis. The mandible (now dated to ∼610 ka) shows features of an earlier human species (Homo erectus) in its size, robusticity, and lack of chin. It also shows features that ally it with later species, such as the relatively small teeth. While initially suggested as an ancestor for the Neanderthals, similarities were later noted between Mauer and remains from Arago (France), which are in turn often grouped with comparable Mid-Pleistocene fossils from Europe and Africa which are candidates for the last common ancestor. If Mauer is included in this group, it would be designated H. heidelbergensis. Where did they live? Putative H. heidelbergensis specimens include African examples such as Bodo (Ethiopia) and Broken Hill (Zambia), European fossils such as Petralona (Greece) and Arago and possibly also Asian specimens such as Dali (China) and Narmada (India). Some argue that the Mauer mandible should not be used to name this larger Mid-Pleistocene group. As the quote (attributed to William Straus) goes: "While the skull is the creation of God, the jaw is the work of the devil." These researchers agree that there are similarities between some of these Mid-Pleistocene fossils, but point out that basing the species definition on an isolated mandible is problematic. Not only is mandibular material lacking for most of the other fossils, but the mandible does not have a large number of taxonomically diagnostic features. If Mauer is excluded from the taxon likely to be the last common ancestor, the name H. rhodesiensis would take priority if the Broken Hill fossil is included. However, a study on Pleistocene and modern human mandibles argues that a suite of diagnostic features can be identified that support the grouping of these Mid-Pleistocene fossils with Mauer, and thus the name H. heidelbergensis is appropriate for the last common ancestor. What is H. heidelbergensis' position in the human family tree? Because of the presence of some claimed Neanderthal traits, such as in the morphology of the teeth and the shape of the face, in European Mid-Pleistocene specimens, some researchers have argued that H. heidelbergensis is the ancestor of Neanderthals to the exclusion of the African fossils and H. sapiens (Figure 1A). In this scenario, similarities between African and European Mid-Pleistocene hominins are merely primitive traits inherited from H. erectus; H. sapiens is descended from a separate African Mid-Pleistocene species (possibly H. rhodesiensis), and the last common ancestor of H. sapiens and Neanderthals would lie chronologically further back, perhaps in the form of H. antecessor, thus far only recognised from the site of Gran Dolina at Atapuerca (Spain) at ∼800 ka. The hypothesis of H. heidelbergensis as uniquely ancestral to the Neanderthals is highly dependent on the inclusion of the remains from another site at Atapuerca, the controversial Sima de los Huesos ('Pit of the bones'), which is where most of the fossils with claimed Neanderthal affinities come from. The more than 6000 fossils from that site show distinctive Neanderthal features, but have often been included in the H. heidelbergensis taxon because of their supposed great age (up to 600 ka). Further research has now suggested that the material looks too Neanderthal and is too young (∼400 ka) to represent H. heidelbergensis, making these fossils early Neanderthals instead. However, adding extra complexity, recent findings reveal the Sima people's mitochondrial DNA to be more similar to that of the Denisovans (see below) than Neanderthals. Are they our ancestors? African H. heidelbergensis material, such as Broken Hill, shares numerous features with European fossils such as Petralona, leading many to group them together. As long as Mauer is also included, this taxon can be named H. heidelbergensis. Proponents of this wide concept of H. heidelbergensis assert that the mosaic of primitive and derived features shared by this group of fossils is unique, with few traits linking them exclusively to either modern humans or Neanderthals (Figure 1B). H. heidelbergensis is thus hypothesised to be the last common ancestor of both Neanderthals in Eurasia and H. sapiens in Africa. This scenario is probably the most popular and well supported at present. Why does it matter? Modern humans and Neanderthals show clear behavioural and morphological differences from each other, but also clear similarities. Determining the tempo and mode of the evolution of their distinct features requires an estimate for the divergence date of their lineages from the last common ancestor, while assessing the extent of behavioural and morphological features that may have evolved in parallel (e.g. burial of the dead, neurocranial expansion) requires knowledge of those features in the LCA. As yet, no H. heidelbergensis DNA has been sequenced, so the species cannot be recognised genetically, but by comparing genetic data from Neanderthals and H. sapiens it is possible to estimate a hypothetical divergence date (∼410–440 ka) for the two species, based on mitochondrial DNA, providing calibration for the putative transition from H. heidelbergensis to its daughter species. If we know when and where we diverged from our closest relatives, we also have a better chance of inferring the selective pressures behind the origin of our species, finding out why populations of H. heidelbergensis separated and evolved into H. sapiens and Neanderthals. You mentioned Denisovans. Could they be H. heidelbergensis or its descendants? In 2010, DNA sequenced from a fossil finger bone in Siberia showed the existence of an unexpected additional human population in the late Pleistocene. This group was christened the 'Denisovans' after the site of Denisova Cave. We still don't know what the Denisovans looked like, or completely understand their position relative to other species, but genomic data suggest they are related to the Neanderthals (Figure 1A,B). However, evidence of interbreeding with recent H. sapiens in Southeast Asia shows that the Denisovans were once widespread in the region. As there are potential H. heidelbergensis fossils from Asia, it is possible they could represent the ancestors of the Denisovans. Recent advances in ancient DNA extraction and processing mean that recovering diagnostic genetic material from mid-Pleistocene fossils such as the remains from Asia is now an exciting possibility. The geographic origin of H. heidelbergensis is still unknown, but the early fossils from Asia suggest that continent is as likely a place of origin as Europe or Africa at the moment. An Asian origin for a species directly ancestral to our own would certainly shake up the current rather Afro-centric view of our evolution.