Beyond Homo erectus: A Multi-Wave Model of Early Human Migration

Abstract

For decades, the prevailing narrative of human evolution asserted that Homo erectus was the singular pioneer of the genus Homo, the first to breach the African continent and colonize Eurasia approximately 1.8 million years ago (Ma). This model relied on the assumption that obligate bipedalism, significant encephalization, and advanced social structures were prerequisites for intercontinental dispersal. However, a convergence of recent paleoanthropological discoveries—ranging from the 2.12 million-year-old lithic artifacts of Shangchen, China, to the taxonomic re-evaluation of the Dmanisi hominins in Georgia—has dismantled this unilinear hypothesis. This report synthesizes current research, including the pivotal 2025 dental morphometric studies, to argue for a multi-wave dispersal model. The evidence suggests that the earliest migrants were not the robust Homo erectus but rather smaller-bodied, ecologically flexible populations retaining primitive "mosaic" features, potentially distinct species such as Homo georgicus and Homo caucasi.

1. Introduction: The Collapse of the "Single Hero" (Homo erectus) Narrative

The "Out of Africa 1" event—the initial dispersal of hominins from Africa into Eurasia—has long been depicted as a technological and biological triumph led by Homo erectus. In this traditional view, often termed the "Single Species Hypothesis," Homo erectus (or its African variant Homo ergaster) evolved around 1.9 Ma with a modern body plan, large brain, and the Acheulean handaxe, traits that allowed it to conquer the "Savannahstan" belt stretching from the Rift Valley to Java.¹

However, the fossil and archaeological record has become increasingly defiant of this simplified story. The discovery of the Dmanisi hominins in the 1990s, with their shockingly small brains (approx. 546–775 cc) and primitive upper bodies, presented the first major crack in the erectus armor.³ Were these merely a dwarfed, peripheral population of erectus, or something entirely different?

The situation grew more complex with the 2018 publication of the Shangchen site in China, which pushed the date of human occupation in East Asia back to 2.12 Ma—predating the earliest known Homo erectus fossils in Africa.⁵ Most recently, quantitative analyses published in late 2025 have provided robust statistical evidence that the Dmanisi assemblage is not a single variable species but likely contains at least two distinct taxa.⁷ These findings compel a restructuring of our understanding: the first exodus was likely undertaken by a pre-erectus grade of Homo, utilizing simple tools and exploiting "Green Corridors" during climatic windows of opportunity.

2. The Chronological Disruption: Evidence from Shangchen

The most significant challenge to the timeline of Homo erectus as the primary disperser comes from the Loess Plateau of central China. Before 2018, the Dmanisi site (1.85 Ma) was considered the oldest secure evidence of hominins outside Africa. The excavations at Shangchen, led by Zhaoyu Zhu and Robin Dennell, shattered this ceiling.

2.1 Stratigraphy and the 2.12 Ma Benchmark

Unlike the volcanic ash layers of East Africa that allow for radiometric argon dating, the Chinese Loess Plateau requires magnetostratigraphy—a method that dates sediment layers by correlating their magnetic polarity with the known history of Earth's magnetic field reversals.

At Shangchen, researchers identified a continuous sequence of loess (wind-blown dust) and paleosol (ancient soil) layers. The oldest stone tools were recovered from Loess Layer 28 (L28). Paleomagnetic analysis placed this layer immediately adjacent to the Réunion Subchron, a brief period of "normal" magnetic polarity that occurred within the larger Matuyama Reversed Epoch.⁹

• The Date: This correlation provides an age estimate of approximately 2.12 million years ago.⁶

• The Implication: This date is roughly 270,000 years older than the earliest occupation at Dmanisi and predates the classic appearance of Homo erectus in the fossil record.

2.2 The "Ghost" Toolmaker

No hominin fossils were found at Shangchen, only stone tools. The assemblage consists of simple cores, flakes, scrapers, and points, characteristic of Mode 1 (Oldowan) technology.¹¹ The absence of bifacial handaxes (Mode 2) is consistent with a pre-erectus dispersal.

The identity of the Shangchen toolmaker remains a subject of intense debate. Given the age, the candidates are:

1. Early Homo erectus: Unlikely, as the species had not yet fully coalesced in Africa.

2. Homo habilis: Known from Africa ~2.4 Ma, possessing the manual dexterity for Oldowan tools.

3. A "Ghost" Lineage: A transitional form intermediate between Australopithecus and Homo, which dispersed rapidly across Asia.

The existence of Shangchen proves that hominins capable of surviving in temperate latitudes left Africa long before the evolution of the large-bodied Homo erectus.

3. The Dmanisi Crossroads: Taxonomy and Variation

If Shangchen provides the "when," the site of Dmanisi in the Republic of Georgia provides the "who." Dated to roughly 1.85–1.77 Ma ¹³, Dmanisi preserves a population trapped in time by volcanic ash, offering a unique snapshot of early Pleistocene variation.

3.1 The "Single Species" Controversy

The Dmanisi assemblage includes five well-preserved crania that exhibit a range of variation initially described as unprecedented.

• Skull 5 (D4500/D2600): A massive, prognathic face paired with a tiny braincase (~546 cc), resembling Australopithecus.¹⁴

• Skulls 1–4: More gracile faces with slightly larger brains (up to 775 cc), resembling early Homo erectus or Homo ergaster.³

For years, the dominant hypothesis, championed by David Lordkipanidze, was that these differences represented extreme sexual dimorphism within a single species (Homo erectus ergaster georgicus).¹⁵ They argued the variation was no greater than that seen in modern chimpanzees or humans.

3.2 The 2025 Dental Study: Statistical Rejection of Unity

A groundbreaking study published in PLoS ONE in December 2025 by Nery, Neves, Valota, and Hubbe has effectively challenged the single-species model. The researchers utilized Linear Discriminant Analysis (LDA) to examine the dental crown surface areas of the Dmanisi mandibles and maxillae against a massive comparative dataset of 583 fossil teeth.⁷

Key Findings:

• Cluster Analysis: The study found that the "Skull 5" specimen (D4500-D2600) clustered strongly with australopiths, while specimens D2282 and D2700 clustered with early Homo.⁸

• Dimorphism Test: The researchers compared the Dmanisi variation against male and female gorillas (Gorilla gorilla), the most sexually dimorphic living primate. The variation at Dmanisi exceeded even the extreme dimorphism of gorillas. The probability that the dental differences were solely due to sex was statistically negligible.⁸

Table 1: Proposed Taxonomic Split at Dmanisi (Based on Nery et al., 2025)

Specimen Group	Associated Fossil ID	Morphological Affinity	Proposed Taxon
Group A	D4500 / D2600 ("Skull 5")	Australopith-like dentition, massive mandible	Homo georgicus
Group B	D2282, D211, D2700	Early Homo-like, gracile mandible	Homo caucasi

This supports the hypothesis that Dmanisi was a "crossroads" inhabited by at least two distinct lineages: a primitive remnant population (H. georgicus) and a more derived group (H. caucasi) that may represent a separate migration event or local speciation.⁷

4. Anatomical Mosaics: Walking Human, Climbing Ape?

The confusion over Dmanisi's classification is partly due to the "mosaic" nature of the skeletons—they possess a mix of features that should not exist together according to the classic Homo erectus model.

4.1 Postcranial Paradoxes

The postcranial remains (bones below the skull) reveal a creature that walked efficiently but retained primitive upper-body features.

• The Legs (Derived): The Dmanisi hominins possessed relatively long legs and essentially modern body proportions in the lower limb. The foot structure included a longitudinal arch and an adducted hallux (non-divergent big toe), hallmarks of efficient bipedal striding.¹⁹

• The Arms (Primitive): In stark contrast, the humerus (upper arm bone) lacked humeral torsion. In modern humans and H. erectus, the humeral head is twisted relative to the elbow, allowing the arms to swing naturally at the sides. The Dmanisi humeri are "straight," a primitive trait shared with Homo floresiensis and Homo habilis.¹⁹

• The Shoulder: The scapula (shoulder blade) features a glenoid cavity oriented somewhat cranially (upward), suggesting that these hominins frequently engaged in overhead behaviors, such as climbing or reaching, distinct from the exclusively terrestrial existence of later humans.¹⁹

4.2 Stature Estimates

Body size estimates for Dmanisi range from 145 to 155 cm in height and 40 to 50 kg in weight.²¹ This is significantly smaller than the classic Homo erectus specimen KNM-WT 15000 ("Nariokotome Boy"), who was projected to reach over 180 cm. The small stature of the Dmanisi population further aligns them with H. habilis rather than the "giant" erectus of popular imagination.

Table 2: Comparative Metrics of Early Hominins

Metric	Homo habilis	Dmanisi Hominins	Homo erectus (African)
Cranial Capacity	~610 cc	546 – 775 cc 4	800 – 1000+ cc
Stature	118 – 130 cm	145 – 155 cm 22	160 – 185 cm 23
Humeral Torsion	Absent	Absent 19	Present
Tool Industry	Oldowan (Mode 1)	Oldowan (Mode 1) 12	Acheulean (Mode 2) / Oldowan
Vertebral Morphology	Small, ape-like	Modern-human-like spine 15	Large, derived

5. The Second Wave: Ubeidiya and the Arrival of Erectus

If Shangchen and Dmanisi represent a "First Wave" of primitive, small-bodied hominins, the site of ‘Ubeidiya in Israel represents the arrival of the "true" Homo erectus.

5.1 A Distinct Migration Event

Located in the Jordan Valley and dated to approximately 1.5 million years ago, ‘Ubeidiya yields a different picture of human evolution. In 2022, a study of a child's vertebra (UB 10749) from the site revealed it to be morphologically distinct from the Dmanisi vertebrae. The ‘Ubeidiya bone belonged to a larger-bodied individual, consistent with the robust Homo erectus known from Africa.²⁴

5.2 Technological Shift

The cultural material at ‘Ubeidiya includes Acheulean handaxes (Mode 2).²⁶ This contrasts sharply with the Mode 1 tools of Dmanisi and Shangchen. The appearance of the Acheulean at 1.5 Ma in the Levant, roughly 300,000 years after its invention in Africa, suggests that a new population—biologically and culturally distinct from the Dmanisi peoples—migrated out of Africa in a "Second Wave".²⁷

6. Ecological Gateways: The "Green Corridors"

How did these early, small-brained hominins cross the formidable barrier of the Sahara? Recent paleoclimatic reconstructions have replaced the image of a permanent desert with that of a pulsating "Green Arabia."

• Climatic Windows: Speleothem (cave deposit) dating from the Arabian Peninsula indicates multiple humid phases over the last 2 million years. During these periods, increased monsoon rainfall transformed the desert into grasslands and savannahs, creating "Green Corridors" linking Africa to the Levant.²⁹

• Paleo-Rivers: Reconstructions of ancient river systems suggest that continuous waterways once flowed from the African interior toward the Mediterranean, allowing hominins to migrate without crossing hyper-arid zones.²⁹

• Adaptability: The Dmanisi site itself was not a savannah but a mosaic of Mediterranean forest and steppe.³¹ The ability of early Homo to adapt to these temperate, seasonal environments—and the even colder loess plains of Shangchen—demonstrates an ecological flexibility that predates the biological specializations of Homo erectus.

7. Conclusion: A Bush, Not a Ladder

The hypothesis that Homo erectus was the sole, heroic conqueror of Eurasia is no longer tenable. The evidence from Shangchen (2.12 Ma) and Dmanisi (1.85 Ma) indicates that the first dispersals were undertaken by populations that were biologically primitive—retaining small brains, short statures, and archaic upper bodies. These pioneers, potentially classified as Homo georgicus or late Homo habilis, utilized simple stone tools and exploited climatic greening events to expand their range.

The "classic" Homo erectus, with its large body and Acheulean handaxes, likely represents a secondary wave of migration that occurred hundreds of thousands of years later (circa 1.5 Ma), as evidenced at ‘Ubeidiya. Human evolution outside Africa was not a single arrow shot from the bow of the Rift Valley, but a series of pulses, featuring different species and technologies, weaving a complex tapestry of survival across the Old World.

Citations

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Works cited

1. The Story of Homo erectus at 'Ubeidiya in Israel - Sapiens.org, accessed January 10, 2026, https://www.sapiens.org/archaeology/ubeidiya-homo-erectus/

2. New evidence of the possible coexistence of two separate early human species, accessed January 10, 2026, https://www.wsws.org/en/articles/2026/01/04/tvzi-j04.html

3. Dmanisi hominins - Wikipedia, accessed January 10, 2026, https://en.wikipedia.org/wiki/Dmanisi_hominins

4. Fossil Hominids: Skull D2700 - TalkOrigins Archive, accessed January 10, 2026, https://www.talkorigins.org/faqs/homs/d2700.html

5. Discovery of ancient tools in China suggests humans left Africa earlier than previously thought - University of Exeter, accessed January 10, 2026, https://news-archive.exeter.ac.uk/2018/july/title_669981_en.html

6. Oldest early human toolmakers in Asia - The Smithsonian's Human Origins Program, accessed January 10, 2026, https://humanorigins.si.edu/research/whats-hot-human-origins/oldest-early-human-toolmakers-asia

7. Testing the taxonomy of Dmanisi hominin fossils through dental crown area - PMC - NIH, accessed January 10, 2026, https://pmc.ncbi.nlm.nih.gov/articles/PMC12674552/

8. Testing the taxonomy of Dmanisi hominin fossils through dental crown area | PLOS One, accessed January 10, 2026, https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0336484

9. Oldest Stone Tools Outside Africa Unearthed in China - Smithsonian Magazine, accessed January 10, 2026, https://www.smithsonianmag.com/smart-news/oldest-stone-tools-outside-africa-unearthed-china-180969621/

10. Study on the loess-paleosol sequence of the Dingcun Palaeolithic Sites - ResearchGate, accessed January 10, 2026, https://www.researchgate.net/publication/285899245_Study_on_the_loess-paleosol_sequence_of_the_Dingcun_Palaeolithic_Sites

11. Stone Tools Found in Northwest China Point to Human Life 2.1m Years Ago, accessed January 10, 2026, https://english.cas.cn/newsroom/archive/news_archive/nu2018/201807/t20180713_195113.shtml

12. Palaeoenvironment of Africa and Human Evolution Part IVa - Odyssey: Adventures in Archaeology, accessed January 10, 2026, http://www.odysseyadventures.ca/articles/humanOrigins/article_palaeoenvironment_04.html

13. Earliest human occupations at Dmanisi (Georgian Caucasus) dated to 1.85–1.78 Ma - PMC, accessed January 10, 2026, https://pmc.ncbi.nlm.nih.gov/articles/PMC3127884/

14. New Dmanisi fossil study suggests multiple human species left Africa 1.8 million years ago, accessed January 10, 2026, https://archaeologymag.com/2026/01/dmanisi-fossil-study-multiple-species-left-africa/

15. Homo erectus - the Dmanisi Site - Don's Maps, accessed January 10, 2026, https://donsmaps.com/dmanisi.html

16. (PDF) Response to Comment on “A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo” - ResearchGate, accessed January 10, 2026, https://www.researchgate.net/publication/263611150_Response_to_Comment_on_A_Complete_Skull_from_Dmanisi_Georgia_and_the_Evolutionary_Biology_of_Early_Homo

17. Skull of Homo erectus throws story of human evolution into disarray | Europe | The Guardian, accessed January 10, 2026, https://www.theguardian.com/science/2013/oct/17/skull-homo-erectus-human-evolution

18. Testing the taxonomy of Dmanisi hominin fossils through dental crown area | PLOS One, accessed January 10, 2026, https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0336484

19. (PDF) Postcranial Evidence from Early Homo from Dmanisi, Georgia, accessed January 10, 2026, https://www.researchgate.net/publication/5961309_Postcranial_Evidence_from_Early_Homo_from_Dmanisi_Georgia

20. Postcranial evidence from early Homo from Dmanisi, Georgia, accessed January 10, 2026, https://experts.umn.edu/en/publications/postcranial-evidence-from-early-homo-from-dmanisi-georgia/

21. Long-term patterns of body mass and stature evolution within the hominin lineage, accessed January 10, 2026, https://royalsocietypublishing.org/rsos/article/4/11/171339/93531/Long-term-patterns-of-body-mass-and-stature

22. Stature, body mass, and brain size: A two-million-year odyssey - Cognitive Archaeology, accessed January 10, 2026, https://cognitivearchaeologyblog.wordpress.com/wp-content/uploads/2015/11/2013-gallagher-body-size-homo.pdf

23. Homo erectus, our ancient ancestor | Natural History Museum, accessed January 10, 2026, https://www.nhm.ac.uk/discover/homo-erectus-our-ancient-ancestor.html

24. TU paleoanthropologist uncovers ancient humans in Israel - - The University of Tulsa, accessed January 10, 2026, https://utulsa.edu/news/prehistoric-humans-belmaker/

25. Prehistoric human vertebra discovered in the Jordan Valley tells the story of prehistoric migration from Africa | ScienceDaily, accessed January 10, 2026, https://www.sciencedaily.com/releases/2022/02/220202080314.htm

26. Ubeidiya prehistoric site - Wikipedia, accessed January 10, 2026, https://en.wikipedia.org/wiki/Ubeidiya_prehistoric_site

27. Hominin Bone in Israel Dated to 1.5 Million Years Ago - Archaeology Magazine, accessed January 10, 2026, https://archaeology.org/news/2022/02/07/220208-israel-migration-evolution/

28. The origins of the Acheulean: past and present perspectives on a major transition in human evolution, accessed January 10, 2026, https://royalsocietypublishing.org/rstb/article/371/1698/20150245/22973/The-origins-of-the-Acheulean-past-and-present

29. Rivers and Bones - Open Rivers Journal, accessed January 10, 2026, https://openrivers.lib.umn.edu/article/rivers-and-bones/

30. Green Arabia Hypothesis in Human Evolution Research - CivilsDaily, accessed January 10, 2026, https://www.civilsdaily.com/news/green-arabia-hypothesis-in-human-evolution-research/

31. New paleoecological inferences based on the Early Pleistocene amphibian and reptile assemblage from Dmanisi (Georgia, Lesser Caucasus) | Request PDF - ResearchGate, accessed January 10, 2026, https://www.researchgate.net/publication/356971683_New_paleoecological_inferences_based_on_the_Early_Pleistocene_amphibian_and_reptile_assemblage_from_Dmanisi_Georgia_Lesser_Caucasus

32. Where do the Dmanisi hominins fit on the human evolutionary tree? - bioRxiv, accessed January 10, 2026, https://www.biorxiv.org/content/10.1101/2025.03.01.639363v1.full

33. Postcranial evidence from early Homo from Dmanisi, Georgia - PubMed, accessed January 10, 2026, https://pubmed.ncbi.nlm.nih.gov/17882214/

34. Early human migrations - Wikipedia, accessed January 10, 2026, https://en.wikipedia.org/wiki/Early_human_migrations

35. Rethinking the evolution of the human foot: insights from experimental research, accessed January 10, 2026, https://journals.biologists.com/jeb/article/221/17/jeb174425/19587/Rethinking-the-evolution-of-the-human-foot

36. A discussion on establishing a "loess-paleosol-hominin remains sequence" in the Chinese Loess Plateau - 第四纪研究, accessed January 10, 2026, http://www.dsjyj.com.cn/en/article/doi/10.11928/j.issn.1001-7410.2025.01.01?viewType=HTML

37. (PDF) Just how strapping was KNM-WT 15000? - ResearchGate, accessed January 10, 2026, https://www.researchgate.net/publication/46287881_Just_how_strapping_was_KNM-WT_15000