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The primitive brain of early Homo - Science Magazine
Apr 08, 2021 5 mins, 54 secs
By comparing endocasts, representations of the inner surface of fossil brain cases, from early Homo from Africa, Georgia, and Southeast Asia, they show that these structural innovations emerged later than the first dispersal of the genus from Africa, and were probably in place by 1.7 to 1.5 million years ago.

Their findings suggest that brain reorganization was not a prerequisite for dispersals from Africa, and that there might have been more than one long-range dispersal of early Homo.

Here, we show that the brains of early Homo from Africa and Western Asia (Dmanisi) retained a primitive, great ape–like organization of the frontal lobe.

By contrast, African Homo younger than 1.5 million years ago, as well as all Southeast Asian Homo erectus, exhibited a more derived, humanlike brain organization.

Frontal lobe reorganization, once considered a hallmark of earliest Homo in Africa, thus evolved comparatively late, and long after Homo first dispersed from Africa.

The endocranial region that holds key information about frontal lobe reorganization and possible language capabilities is Broca’s cap (BC), a bulge on the lateral fronto-orbital surface of endocasts (Fig. 1).

However, endocranial evidence from Australopithecus africanus and Australopithecus sediba has been interpreted as showing incipient reorganization of the frontal lobe (10, 11).

(A) In great apes, the precentral sulcus (pc) crosses the coronal suture (CO), such that its inferior portion (pci) is located anterior to CO.

Traditionally, it has been assumed that a derived, humanlike organization of the frontal and parieto-occipital lobes (see legend of Fig. 1) characterizes the genus Homo from its beginnings (21–23).

The fossil hominins from the early Pleistocene site of Dmanisi, dated to 1.85 to 1.77 Ma (27, 28), play a key role in assessing brain reorganization in early Homo and its potential importance for hominin dispersals from Africa.

Here, we track key changes in brain organization of early Homo from ~1.8 Ma onward by analyzing the endocranial morphology of the five Dmanisi specimens and reevaluating an extended comparative sample of African and Southeast Asian fossil endocasts (33) (Table 1).

To identify primitive versus derived aspects of frontal lobe organization in fossil endocasts, we use the highly distinct cranio-cerebral topographies of humans and great apes as a frame of reference (Fig. 1) (34, 35).

A characteristic signal of inferior frontal lobe expansion is the posterior shift of the inferior precentral sulcus (pci) relative to the coronal suture (CO) (35) (Fig. 1).

In addition, we use geometric morphometric methods to identify which endocranial and brain regions underwent differential expansion during frontal lobe reorganization (supplementary methods M4 and M5).

This latter structure originates near the apex of the endocast, crosses the coronal suture at mid-height, and courses toward BC, such that its inferior portion (pci) lies anterior to the coronal suture.

(See also fig. S2.) In all specimens, the precentral sulcus (pc) crosses the coronal suture (CO), such that its inferior portion (pci) lies anterior to CO, indicating a primitive organization of the frontal lobe (see Fig. 1A).

The inferior portion of the precentral sulcus (pci) lies in front of the coronal suture and courses toward BC.

Altogether, the Dmanisi endocasts indicate a consistent topographic pattern of external cortical morphology, where the precentral sulcus crosses the coronal suture such that its inferior portion is anterior to the suture and courses toward BC (Fig. 2, Table 1, and fig. S1).

A comparative analysis of the endocasts of early to mid-Pleistocene Homo fossils from Africa reveals greater endocranial topographic diversity (Table 1, supplementary text S2, and fig. S2).

Specimens KNM-ER1805 and KNM-ER1813, with disputed taxonomic affiliation but dated to before ~1.7 Ma, show endocranial imprints that are compatible with a great ape–like organization of the frontal lobe (fig. S2, A and B).

Specimen KNM-ER 1470 (fig. S2C), which has been put forward as key evidence for a derived frontal lobe organization (21), is too fragmented to reveal diagnostic endocranial imprints.

erectus, show variation in frontal endocranial topography that suggests a range of primitive to derived frontal lobe morphologies (fig. S2, D to F).

By contrast, African fossils younger than ~1.5 Ma exhibit endocranial topographies that are compatible with a derived, modern human–like frontal lobe organization (Table 1 and supplementary text S2).

In all specimens reexamined here, imprints of the inferior and/or middle frontal sulci course anteroposteriorly to reach the coronal suture (Table 1, supplementary text S3, and fig. S3, A to E).

This topography constrains the inferior precentral sulcus to a location that either roughly coincides with the coronal suture or is posterior to it, thus representing a derived organization of the frontal lobe.

Correlating our data about endocranial topography (Table 1) with geological dates permits inferences on the timing of frontal lobe reorganization and its possible correlation with brain expansion and changes in brain shape (Fig. 4).

During that time interval, frontal lobe reorganization occurred in tandem with brain expansion (Fig. 4A and Table 1) [mean endocranial volume (ECV) increased from 650 to 830 cm3].

naledi (9) (supplementary text S4), suggests that frontal lobe organization is not mechanistically linked to large brain volumes.

Orange, gray, and turquoise symbols indicate the position of the inferior precentral sulcus (pci) anterior to, coincident with, and posterior to the coronal suture (CO), respectively (see inset graph).

(B) Endocranial shape change associated with frontal lobe reorganization (left lateral view).

Colors denote above-average expansion (green) and bulging (yellow) of the endocranial surface, indicating differential enlargement of labeled brain regions (IPF, inferior prefrontal cortex; 45/47, Brodmann areas 45/47; PP, posterior parietal cortex; T, temporal lobe, O, occipital lobe).

Our data further indicate that the transition from primitive to derived frontal lobe organization was accompanied by specific changes in endocranial shape (Fig. 4B and supplementary method M5).

Cranio-cerebral topography reveals that the earliest members of the genus Homo had a primitive frontal lobe organization, featuring an ape-like anterior location of the inferior precentral sulcus relative to the coronal suture (Fig. 4A and Table 1).

Our data indicate that the derived frontal lobe organization emerged relatively late during the evolution of Homo, between 1.7 and 1.5 Ma—not at the transition from Australopithecus to Homo (10, 39), but clearly later than the first dispersals of Homo from Africa.

Endocranial shape change associated with frontal lobe reorganization reveals differential expansion of the inferior prefrontal cortex and also of the posterior parietal and occipital cortex (Fig. 4B) (39).

We infer from this that endocasts of early Homo predating frontal lobe reorganization potentially exhibit imprints of remnant ape-like lunate sulci in the parieto-occipital region (supplementary text S1 and fig. S1).

The temporal and geographic patterning of primitive and derived brain organization in early Homo (Table 1) cannot be explained by a single-dispersal scenario but must have involved greater spatiotemporal complexity, as suggested earlier (42–45).

Given the current evidence (Fig. 4 and Table 1), the most parsimonious scenario is that the first Homo populations to disperse from Africa, probably as early as 2.1 Ma (46), retained the primitive frontal lobe organization, as represented in Dmanisi.

The latter view is supported by the observation that metric variation in gnathic morphology is greater than expected for a single taxon, suggesting dietary specialization rather than brain expansion as a major driver of speciation in early Homo (56).

Early Homo evolved a modern humanlike brain organization only after its first dispersal from Africa.

Early Homo evolved a modern humanlike brain organization only after its first dispersal from Africa

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