Ancient DNA Analysis of I7632 and I14551: Middle Iron Age Individuals from Slonk Hill, Brighton, Sussex

I7632 and I14551 are two Middle Iron Age individuals recovered as inhumations from the same enclosure at Slonk Hill, Brighton, Sussex, England, and identified genetically as 2nd–3rd degree biological relatives.

I7632 (Grave 1) was directly radiocarbon dated on human bone collagen to 2246 ± 30 BP (SUERC‑70740), calibrating to 391–203 cal BCE on the IntCal20 terrestrial curve.

I14551 (Grave 2) was independently dated on human bone collagen to 2333 ± 31 BP (SUERC‑76365), calibrating to 514–234 cal BCE on the same curve.

The two calibrated ranges overlap closely, and the established kinship between the individuals confirms that both burials belong to the same coherent Middle Iron Age horizon.

Although Grave 1 contained a mussel-shell layer, the agreement between the two collagen dates demonstrates that no marine reservoir offset affected the determination for I7632, and no dietary indicators in the dataset suggest marine consumption sufficient to distort either calibrated age.

Genetically, both individuals fall within the England and Wales_IA autosomal cluster, also referred to as England_MIA in regional breakdowns, as defined through supervised ADMIXTURE, PCA, and qpAdm modelling reported in the study, and neither is identified as an outlier.

Their ancestry fits the standard Iron Age British mixture of Early European Farmer, Western Hunter-Gatherer, and Steppe ancestry, with acceptable qpAdm model fit (p > 0.01).

I7632 is male and carries Y-chromosome haplogroup R‑S1200, placed phylogenetically along the line R1b‑M269 → L23 → L51 → P310 → S1194 → S1200.

His mitochondrial haplogroup is H1.

I14551 is female,  her mitochondrial haplogroup is H6a1a.

Kinship analysis using READ and lcMLkin, as described in the genetic methods section of the study, confirms the pair as 2nd–3rd degree relatives, consistent with their shared archaeological and chronological context.

Both genomes satisfy the laboratory quality criteria for inclusion in the study.

Libraries were UDG-treated, both individuals exhibit characteristic ancient DNA damage patterns, and contamination estimates fall within acceptable thresholds for genome-wide and uniparental analyses.

Coverage is sufficient for confident autosomal cluster assignment, Y-chromosome haplogroup calling in I7632, and mitochondrial haplogroup assignment in both individuals.

At site level, no additional relatives beyond this pair were detected at Slonk Hill, and no separate migration or genetic signal is reported for the site in the published analyses. Together, I7632 and I14551 represent a small, genetically coherent Middle Iron Age kin-group contributing to the broader genetic profile documented for Iron Age southern Britain.

All radiocarbon determinations, kinship assignments, haplogroup calls, autosomal cluster assignments, and laboratory quality metrics derive from the supplementary data and methods of 

Patterson et al., Nature, 2021/2022 (vol 601), "Large-scale migration into Britain during the Middle to Late Bronze Age"

The supplementary material describes the burial:

Slonk Hill, Brighton, Sussex, England, UK

Rescue excavations on Slonk Hill, Brighton were undertaken by Brighton and Hove Archaeological Society in 1968–74 in advance of the building of a bridge over the River Adur. The excavations uncovered a small enclosed Iron Age settlement with occupation spanning the sixth–first centuries BCE and a later Romano-British settlement spanning the first/second–fourth centuries CE, incorporating two earlier Bronze Age barrows, as well as two Iron Age burials (Graves 1 and 2). Grave 1 comprised an oval-shaped storage pit containing the complete articulated skeleton of a male aged c. 24 years, flexed on his left side with his head to the north and his right hand in front of his face. The body had been placed on top of a layer of shells, mainly mussels, but also winkles and barnacles, which covered the bottom of the pit. The human remains were accompanied by a flint ‘Shepherd’s crown’ (a fossilised sea urchin) and some fragments of pottery dating typologically to the Iron Age. Further potsherds and the shaft of a small iron implement were recovered from the grave fill. 

Grave 2 comprised a small purpose-built grave containing the skeleton of a female aged 35–45 years, buried flexed on her left side with her skull to the north, facing east. A shale bracelet was found on the left forearm and an involuted iron brooch was recovered from near her shoulder. The lower 23cm of grave fill included part of a quern stone and the right half of an ox sacrum.

The skeletons from Grave 1 and Grave 2 produced Middle Iron Age radiocarbon dates of 2246±30 BP (SUERC-70740; 391–203 cal BCE) and 2333±31 BP (SUERC-76365; 514–234 cal BCE), respectively. A tooth from the individual in Grave 1 (HATMP100001) yielded sample I7632 (male); while a petrous bone from the individual in Grave 2 (HATMP100348.2) yielded sample I14551 (female). aDNA analysis revealed that these two individuals are second or third degree relatives.

Phylogenetic Y-DNA analysis by FamilyTreeDNA. Ancient DNA samples are typically degraded and missing coverage, sometimes resulting in less specific haplogroup placements.