Sarmiento and colleagues questioned the legitimacy of H. antecessor as a separate species because much of the skull anatomy is unknown; H. heidelbergensis is known from roughly the same time and region; and because the type specimen was a child (the supposedly characteristic features could have disappeared with maturity.) Such restructuring of the face, they argued, can also be caused by regional climatic adaptation rather than speciation. In 2009 American palaeoanthropologist Richard Klein stated he was skeptical that H. antecessor was ancestral to H. heidelbergensis, interpreting H. antecessor as "an offshoot of H. ergaster [from Africa] that disappeared after a failed attempt to colonize southern Europe". Similarly, in 2012, British physical anthropologist Chris Stringer considered H. antecessor and H. heidelbergensis to be two different lineages rather than them having an ancestor/descendant relationship. In 2013, anthropologist Sarah Freidline and colleagues suggested the modern humanlike face evolved independently several times among Homo. In 2017 Castro and colleagues conceded that H. antecessor may or may not be a modern human ancestor, although if it was not then it probably split quite shortly before the modern human/Neanderthal split. In 2020 Dutch molecular palaeoanthropologist Frido Welker and colleagues concluded H. antecessor is not a modern human ancestor by analysing ancient proteins collected from the tooth ATD6-92. Anatomy Skull The facial anatomy of H. antecessor is predominantly known from the 10–11.5-year-old H. antecessor child ATD6-69, as the few other facial specimens are fragmentary. ATD6-69 is strikingly similar to modern humans (as well as East Asian Middle Pleistocene archaic humans) as opposed to West Eurasian or African Middle Pleistocene archaic humans including Neanderthals. The most notable traits are a completely flat face and a curved zygomaticoalveolar crest (the bar of bone connecting the cheek to the part of the maxilla that holds the teeth). In 2013 anthropologist Sarah Freidline and colleagues statistically determined that these features would not disappear with maturity. H. antecessor suggests the modern human face evolved and disappeared multiple times in the past, which is not unlikely as facial anatomy is strongly influenced by diet and thus the environment. The nasal bones are like those of modern humans. The mandible (lower jaw) is quite gracile unlike most other archaic humans. It exhibits several archaic features, but the shape of the mandibular notch is modern humanlike, and the alveolar part (adjacent to the teeth) is completely vertical as in modern humans. Like many Neanderthals, the