«Every time I see the name ‘Homo heidelbergensis’ I feel a little queasy», John Hawks
Homo heidelbergensis was defined in 1908 as a new species for a mandible that was found one year before, by the Neckar river in Mauer, near Heidelberg in Germany. This mandible, dated to 600 Ka, was the oldest hominin fossil in Europe for the following 90 years.
In the meantime, the name Homo heidelbergensis remained with no further assignment to any other fossil for seven decades, until it was resurrected to try to classify a group of 20+ specimens of the Middle Pleistocene from dispersed sites in Europe (Arago in France, Petralona in Greece…), Africa (Kabwe in Zambia, Bodo in Ethiopia…) and Asia (Yunxian and Dali in China…). They all had in common some derived features from Homo erectus, basically a larger brain which reflects in complex tools (e.g. the wooden spear fron Schöningen, Germany).
«In reality this species should have stayed dead instead of being resurrected in the 1980s», Juan Luis Arsuaga
They were ‘archaic Homo sapiens’, fossils dated to between 600 Ka and 200 Ka just before the Homo sapiens appeared in Africa. It was made necessary to assign them to a species which demonstrated an evolutionary path between erectus and modern humans, being also ancestor of neandertals. Homo heidelbergensis was the choosen name, although there was not any complete description of this species.
Some issues with this species
1) The specimens mentioned above individually share some morphological features with their fellow members, but not with all. And they also share some features with different species from previous and subsequent time lines.
«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», Chris Stringer
2) Starting in the 1990s, 7,000+ hominin fossils dated to 430 Ka have been found at Sima de los Huesos in Atapuerca. They constitute more than 80% of the total hominin fossil record of the Middle Pleistocene. They were initially assigned to Homo heidelbergensis, but later the paleogenetics analysis demonstrated that they have a close relationship with Homo neanderthalensis. Therefore, in 2014 the Atapuerca team argued that those hominins do not belong to H. heidelbergensis.
3) One classic position used to be the consideration of Homo heidelbergensis as the common ancestor of Homo sapiens and Homo neanderthalensis. The relationship with H. neanderthalensis could be related to their robusticity, large brain case and facial features. For example, most Middle Pleistocene mandibles share some neandertal derived features. However some dental morphological traits are different from those in neandertals. Alternative hypotheses place Homo heidelbergensis as a Western European niche, while the African fossils are assigned to Homo rhodesiensis.
4) The type specimen, the Mauer mandible, has not actually typical features of the rest of supposed representatives of the species. Some researchers see the Mauer mandible a unique specimen, while others defend some similarities in the teeth between Mauer and three partial mandibles from Arago.
«While the skull is the creation of God, the jaw is the work of the devil». This anonymous quotation was used by Chris Stringer to refer to the Mauer mandible (and also by Yoel Rak to refer to the two mandibles from the Tabun cave)
Many researches actually recognize a number of features in the Mauer mandible as neandertal-like. In particular, in the ESHE 2016 meeting Rosas argued that the neandertal lineage should include the Mauer mandible: when comparing Mauer with Arago and Sima mandibles, they share common patterns such as the broad rami, the preangular notch and the position of the mental foramen.
So, what’s up with the Homo heidelbergensis name?
“The species H. heidelbergesis should be removed of the hominin phylogeny, and the European Middle Pleistocene specimens could be tentatively included in two or more branches of the Neandertal clade”, José María Bermúdez de Castro
According to the most recent paleogenetics studies, the split between the neandertal and the modern human lineages occurred between 550-765 Ka. The Mauer mandible dated to 600 Ka is the oldest Homo heidelbergensis, and other specimens traditionally assigned to H. heidelbergensis are more recent than that. Both considerations imply that H. heidelbergensis cannot be the common ancestor of neandertals and modern humans.
Sima de los Huesos specimens are now considered as early neandertals. The Altamura skeleton was initially assigned to heidelbergensis until its DNA showed it is a neandertal. Other specimens could likely follow the same neandertal-assignment path in the next few years…
Hominins during the European Middle Pleistocene
The settlement of the European continent is a passionate topic. The samples show a large amount of variability but, once the neandertal morphology pattern is achieved, then a lower level of variation in observed. There are many questions to solve. Was the variation a consequence of several events of first bottlenecking and then hybridization, or both? What was the scale of such events, wide or local? The drastic climatic changes can probably explain scenarios of isolation of some of the populations, the extinction of many, and the hybridization of the remaining.
A recent work (Bermudez de Castro et al, 2016) studies the mandibular features of several specimens from 4 key European Middle Pleistocene sites (Sima de los Huesos, Gran Dolina TD6-2, Arago, Mauer), together with the paleogenetics results and the archeological and paleoenvironmental evidences of this period. The conclusions are:
- A probable demographic discontinuity occurred between the late Early Pleistocene populations c. 800 Ka and the Middle Pleistocene populations c. 600 Ka.
- The generalized and primitive morphology of the TD6-2 mandibles suggest no direct relationship between Homo antecessor and the European Middle Pleistocene hominins. These specimens share a derived neandertal pattern (Mauer, Arago, and Sima de los Huesos mandibles), not present in Gran Dolina.
- However all of them may have had a common ancestor during the late Early Pleistocene. Skeletal parts from TD6 suggest some phylogenetic relationship with the Middle Pleistocene European hominins and the classical neandertal. The morphology recalls some features from Southwestern Asia specimens. That common ancestor could have evolved there, and then an early dispersal into Europe could have resulted in the Homo antecessor found at TD6-2.
- This is also compatible with the above-mentioned paleogenetic results showing the split of the neandertal and modern human lineages between 550-765 Ka. This event may have also occurred from that common ancestor.
- During this transition, for 200,000+ years the climate conditions may have resulted in several groups’ extinction. Meanwhile, new migratory flows into Europe may have taken place, as well as hybridization between the remaining groups which explains the morphological similarities.
More information: Continuity versus discontinuity of the human settlement of Europe between the late Early Pleistocene and the early Middle Pleistocene. The mandibular evidence (Bermudez de Castro et al, 2016) [access].
The slideshow above ‘The controversial Homo heidelbergensis’ is available to download [here].