When the skulls of Homo erectus were discovered in Java, Indonesia, and Beijing, China ~100 years ago, they cemented the notion that modern humans evolved from these Asian hominins. They had relatively big brains, made stone tools, and were upright creatures. There was no similar fossil in Africa until a skull from Kobi Fora was discovered in 1975 and assigned to a new species which was named Homo ergater ("working" man) it was erectus-like (but differed from it), and it was old. This boosted the position of the African origin of humans, so it fitted neatly into the Out Of Africa (OOA) theory. The erectus of Asia were assigned to a branch named Homo erectus sensu stricto, or strict-sense erectus, the classic fossils. And they, together with the African ergasters were grouped as sensu lato (broad sense) erectus.
However, there has been a controversy ever since, and it is likely that they are not related, and that ergaster vanished in Africa without leading to the ancestors of modern human beings.
Antón 2014 summarizes the situation as follows: "Proponents generally allocate early African Homo erectus fossils dating from c. 1.8–1.4 Ma to Homo ergaster, mainly on the basis of smaller, more gracile crania and a lack of robust cranial suprastructures. In this schema, the geographic distribution of Homo erectus sensu stricto is restricted to Asian localities with a few examples (e.g., OH 9) of back migration into Africa. Homo ergaster includes the early African and Georgian specimens attributed by others to H. erectus and is therefore posited as the ancestor to Homo erectus, Homo heidelbergensis, and Homo sapiens..." This quote mentions an "Into Africa" back migration of Eurasian erectus, assigning an OH 9 fossil to this event. Remarkable!
The OH 9 sample was discovered by Louis Leakey in Olduvai Gorge, Tanzania, in 1960. It is approximately 1.4 million years old and has a large brain. In fact, it is the oldest hominin discovered until now, with a braincase capacity above on litre (it is 1,067 cm3). Leakey himself recognized it as erectus, calling it "Chellean man" because in the same sedimentary layer he uneaerthed Acheulean-like stone tools that are the hallmark of Homo erectus. Yet, it is unlike the ergaster fossils, and more or less overlaps with the last of them.
So, according to Antón 2014, ergaster is more gracile (smaller, slender) and the ancestor of the Dmanisi specimens in the Caucasus and ancestor of the sensu stricto erectus, implying that ergaster migrated out of Africa and originated the Homo georgicus found in Dmanisi, and the later erectines. However, it may be possible that the situation is exactly the opposite: that Homo ergaster descends from a Eurasian lineage of hominins, that originated the Dmanisi and erectines, and also migrated into Africa, leading to the hominins there, like Homo habilis, Homo rudolfiensis, and later, Homo ergaster. Real erectus originated and evolved in Asia, and migrated into Africa much later.
The Eurasian origin of Homo erectus
Chris Stinger in an interview (see it online here) suggests that the oldest hominins may not even be homo, they were something else, below I transcribe the relevant part, starting at 35' 45" in the video:
"...I mean it depends again what we call homo erectus. So on our Yunxian paper we obviously analyzed some of these earlier skulls and for us there is a clear erectus group based on the original fossils from Java and the ones from China. They are the real erectus if you like and then there are earlier things in Africa and in Dmanisi in Georgia. Those things on our analyses are probably not actually Homo erectus. They are something different. I mean, you know, we couldn't solve everything in our paper and say what they were, but we think that they are maybe not what you could call real erectus. There's another name around called Homo ergaster, workman, work human. Um, which was given to some fossils from Africa. Um, so sometimes called African Homo erectus, but also Homo ergaster. So that name is also around and could be applied to some of these earlier things. They would still be homo and therefore human in my view. But yes, there might be an earlier species before erectus. Um and in fact there might be even more than one species. So the Dmanisi material from Georgia 1.8 million years old that's been called the first Homo erectus out of out of Africa. Um some people and I'm beginning to join them think that that isn't really Homo erectus. It's something more primitive. Uh and that is the sort of creature that might have given rise to, you know, Homo floresiensis eventually, over in Flores or Homo luzoinensis eventually, over in the Philippines. So we could be looking at something that actually is prehuman even though at the moment there's a homo name attached to it. Some of those things could turn out to be prehuman. So for me, Homo erectus as we know it from Asia is is the first definite human for me and it dates back to you know one and a half million years probably. And then there are some things before where it's tricky to know whether they really are over the line to be homo. Some of them are still small-brained. Some of them still had adaptations in their skeleton which shows that they were either spending time in the trees or they had those remains still in their skeleton showed the traces of tree living. Whereas erectus for me as we know it later on is the first committed ground living in completely you know given up life in the trees and fully committed to life on the ground walking on two legs. So that for me is a big step forward and humans have done that. Some of these earlier things at around 2 million maybe are still in that process of transition."
Stringer mentions the paper The phylogenetic position of the Yunxian cranium elucidates the origin of Homo longi and the Denisovans co-authored by himself and Feng et al., 2025. He is quite categorical about Dmanisi, and ergaster as being prehuman, or, not homo. They are possibly the same creature that originated the Flores Island hobbit or the Luzon hominins.
Ergaster
The label "Homo ergaster" has been assigned to a collection of fragmentary skulls, and assimilated to the erectine group. But these African "erectus" are different from the genuine Asian ones. They are more dimorphic (Sexual dimorphism is when males and females of the same species have different features, like colors, markings, and importantly in the case of primates, body sizes). Large, aggressive males can control more females by fending off rivals if they are bigger bodied. Modern humans are far less dimorphic as we don't rely on aggressive behavior but on cooperation between males. Asian erectus also show a lower dimorphism in comparison to African hominins.
Villmoare 2019 explored the dimorphism of erectines analyzing footprints and found African ones to be futher from modern human sizes, and Asian ones to be closer:
"Here we present an alternative, using footprints from near Ileret, Kenya, to assess the sexual dimorphism of presumptive African Homo erectus at 1.5 Ma... Our results indicate that East African Homo erectus was more dimorphic than modern Homo sapiens, although less so than highly dimorphic apes... As a species, H. erectus shows considerable temporal and geographic variation and, as Plavcan notes, only the African sample (in which he includes Dmanisi) shows a sufficiently high degree of variation to falsify the hypothesis that the species had acquired the relatively monomorphic human-like condition. In his analysis, the high variation in the African sample was driven by the small size of KNM-ER 42700 and the large size of OH 9 (which are separated geographically by 800 kilometers, and temporally by more than 300,000 years), although the Dmanisi sample also increases the variation within H. erectus considerably. The Chinese and Javan samples show a level of variation in cranial measures very similar to that seen in modern humans. The different patterns in the African and Asian samples raise the possibility of a change in patterns of dimorphism, and potentially social behavior, in the H. erectus lineage over time and/or across different geographic regions."
So, about 1.5 million years ago, the "erectus" in Africa had a high level of dimorphism, large males, smaller females. Could we be identifying the remains of males and females as belonging to different species? In other words, could these African hominins assigned to two different species belong to a single species with high sexual dimorphism?
The gracile bones assigned to Homo habilis and Homo rudolfiensis roughly 1.3 m tall (4.3 ft.) and weighing around 35 kg (70 lbs) could have been the females and the larger H. ergaster measuring 1.8 m (6 ft) and weighing 60 kg (130 lbs) could have been the males. Their brain sizes were very similar, smaller than that of genuine erectus:
Bolhuis, 2014 includes the following image; notice how the ergaster brain size clusters close to Homo habilis/Homo rudolfensis:
Did this Homo ergaster - rudolfiensis - habilis survive in some parts of Africa to evolve into the Homo naledi? A branch that survived in South Africa for over one million years, living as recently as 300 ky ago.
The stone tools
Acheulean hand axes are the typical stone tool industry of Homo erectus, however the ergasters of Africa were not so clear cut. Semaw et al., 2020 wrote a research paper with the suggestive title "Co-occurrence of Acheulian and Oldowan artifacts with Homo erectus cranial fossils from Gona, Afar, Ethiopia" where African "erectus" (i.e. Homo ergaster") made more primitive Oldowan tools. Their comments are shown below. But notice how the mistake genuine erectus that back migrated into Africa, from Busidima, with the smaller, gracile African "ergasters" from Dana Aoule North. They mistakenly attribute the differences to dimorphism:
"We report that both Acheulian and Oldowan artifacts and Homo erectus crania were found in close association at 1.26 million years (Ma) ago at Busidima North (BSN12), and ca. 1.6 to 1.5 Ma ago at Dana Aoule North (DAN5) archaeological sites at Gona, Afar, Ethiopia. The BSN12 partial cranium is robust and large, while the DAN5 cranium is smaller and more gracile, suggesting that H. erectus was probably a sexually dimorphic species. The evidence from Gona shows behavioral diversity and flexibility with a lengthy and concurrent use of both stone technologies by H. erectus, confounding a simple “single species/single technology” view of early Homo"
The smaller ergasters used the more primitive Oldowan tool kit. They were not erectus.
The Asian erectus that migrated back into Africa, evolved there as reported by Asfaw et al., 2002 who studied a 1 million year old skull known as Daka: "A hominid calvaria and postcranial remains recently recovered from the Dakanihylo Member of the Bouri Formation, Middle Awash, Ethiopia... These ∼1.0-million-year (Myr)-old Pleistocene sediments contain abundant early Acheulean stone tools... Here we report that the ‘Daka’ calvaria's metric and morphological attributes centre it firmly within H. erectus. Daka's resemblance to Asian counterparts indicates that the early African and Eurasian fossil hominids represent demes of a widespread palaeospecies. Daka's anatomical intermediacy between earlier and later African fossils provides evidence of evolutionary change. Its temporal and geographic position indicates that African H. erectus was the ancestor of Homo sapiens." So the Daka person is more evolved than the older ergasters, and is the ancestor of modern humans.
Pearson, 2021 noted the differences between ergaster and the Asian erectus; they took a sample of specimens (a hypodigm), and usied Computed Tomography to analyze the brain case, such as the middle cranial fossa (MCF), which hints at the development of the temporal lobe, which is linked to memory, language and social skills and cognition:
"A total sample (n = 51) of cranial Computed Tomography scans (CT) from extant H. sapiens and fossil specimens attributed to African Homo ergaster and Javanese H. erectus were generated into three-dimensional (3D) virtual models with three linear metrics digitally measured on the MCF and two metrics to approximate cranial size...
relative MCF proportions showed extant H. sapiens had a larger MCF to relative to cranial size, while specimens attributed to Javanese H. erectus which had a relatively longer and wider MCF compared to African H. ergaster. These findings indicate that despite sampling limitations, the fossil sample did not show differences in allometric scaling compared to extant H. sapiens, but these specimens attributed to Javanese H. erectus and African H. ergaster share the same total cranial size but had different MCF proportions relative to cranial size. The paleoneurological implications are that these specimens attributed to Javanese H. erectus had longer and wider temporal lobes relative to total brain size compared to African H. ergaster suggesting different patterns of cerebrocranial organization within the Homo erectus hypodigm."
The Asian erectus were therefore smarter than the ergaster lineage.
Regarding the African fossils themselves, Tattersall, 2007 cautiously points out that they are many differences between the specimens, highlight is mine: "However, while this separation at the species level of the African and Indonesian hominids is certainly justified, the species H. ergaster as thus constituted still embraces a bewildering morphological variety. Indeed, although this grouping of African fossils seems to form a fairly coherent clade, it also appears quite diverse."
Conclusion
The ergaster line is more primitive than the Asian erectus, it has smaller molars and premolars, a less specialized skull. Add this to the difference in cognition, tool kits (Oldowan vs. Acheulean) and the timelines, and you can only conclude that they are different species and not a single pan-African-Eurasian group.
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