Fossil and genetic evidence suggests that anatomically modern humans originated in Africa and dispersed into Europe around 40,000-50,000 years ago, at the start of the Late Stone Age. Human remains from a site in Lebanon called Ksâr ’Akil have led scientists to hypothesize that modern humans used the coast of the Mediterranean to spread from Africa into Europe.

Few fossils of modern humans dated to this time period have been found, however; furthermore, remains are geographically limited to a few sites: Ksâr ’Akil and Manot Cave, located in the eastern Mediterranean, and Oase, located in Romania. This scant fossil record has proved difficult to interpret, leading to imprecise chronologies of human movement.

Adding to these uncertainties, archaeologists recently found ornamental shell beads in Europe that predate their first occurrence in Levant, the area of the Mideast that borders on the Mediterranean. If evidence from the Late Stone Age in Europe actually predates finds in the Levant, it raises serious questions about whether modern humans passed along a Levantine corridor on their way to colonize Europe.

Since then, however, fossils were discovered in Manot Cave (located in Israel) that place modern humans in the Levant much earlier than previously thought—49,200-60,200 years ago. However, the fossils were located on a natural shelf that was unconnected to other areas of the cave, making it very difficult to accurately date them. Perplexed by the opposing findings and uncertain data, archaeologists decided to reexamine certain sites to piece together the timing of the migration from Africa to Europe with more confidence.

Ksâr ’Akil is a key site located on the Lebanese coast where modern human remains have previously been found in layers that start at the very beginning of the Late Stone Age. Fossils found in the bottom layer of the deposit (human remains called “Ethelruda”), were originally interpreted as having features that grouped them with archaic humans. However, reexamination suggests that the fossil features are consistent with modern humans and are within the range of modern human variation.

Stone tools found within the bottom deposit layer have features associated with the beginning of the Late Stone Age. Within the early Late Stone Age deposit layers, the remains of an eight-year-old human (“Egbert”) and potentially a second individual have been found. Stone tools recovered from these deposits differ from those at the bottom layer, exhibiting features commonly associated with this era.

A team of researchers recently decided to try to associate the site's layers with climate events to provide more reliable dating. At this site, shells from marine, terrestrial, and freshwater species from a variety of layers were recovered during the 1930s and 1940s. The shells were thought to have been collected for consumption or for use as tools or ornaments. By dating Phorcus turbinatus shells, archaeologists were able to build a concise timeframe of mollusk collection and consumption.

Sixteen samples from the Ksâr ’Akil Late Stone Age sequence were radiocarbon dated. Phorcus turbinatus was found in deposit layers attributed to the beginning of the Late Stone Age and is dated from 43,100-44,400 cal BP (calibrated years before present, a term indicating that the dates come from radiocarbon calibration using tree ring data). The dates from the layers of the early Late Stone Age range from 37,200 to 44,000 cal BP. Finally, the layers from the latter periods of the Late Stone Age date to 31,700 to 40,700 cal BP.

The age of the fossils within the layers should decrease as we move upward through the layers toward the surface. However, previous dating of certain layers fit poorly into the overall sequence, as they appear to have younger ages than the layers above them. This type of inconsistency can be attributed to contamination of the sample, which would result in a younger estimate than the actual sample age. To overcome the limitations associated with characterizing layers of deposits, the team decided to investigate the integrity of individual shells.

First, mathematical modeling was used to assess the accuracy of the previously reported dates. The model indicated that the older dates of the early Late Stone Age best reflected the true ages of the deposits. The model was also applied to the layers containing human remains. The layer containing the remains of the eight-year-old boy “Egbert” was found to have an age of 42,900 to 43,200 cal BP. The layer containing the remains of the “Etherulda” remains could not be properly dated; however, the surrounding layers were found to have an age of 44,100 and 44,600 cal BP. This indicates that the minimum age for the specimen “Etherulda” was 45,900 cal BP.

Additionally, the team studied the extent of racemization of amino acids in the shell samples. Amino acids—the building blocks of proteins—exist with a unique three-dimensional orientation, which is often called “handedness.” This “handedness” can be reversed over time due to different chemical reactions, a process called racemization. Analysis of proteins from the shell Phorcus turbinatus reveals that the shells were not compromised after deposition during solidification, supporting the dates these archaeologists determined.

The team also studied oxygen isotopes in the samples. These values were converted to Sea Surface Temperatures (SST) to observe fluctuations in mean annual SST, providing critical climate information. Oxygen isotope and SST data were found to be consistent with the calibrated ages. Samples containing higher values of the oxygen isotopes corresponded to colder events where the SST was between 19.2 degrees Celsius and 21.4 degrees Celsius. Samples containing lower values of the oxygen isotopes corresponded to warmer events where the SST was between 23.2 degrees Celsius and 24.4 degrees Celsius.

One specimen was found to have inconsistent amino acid racemization and oxygen isotope levels, indicating that this specimen contaminated the deposit and came from a colder period.

Overall, this study provides a thorough analysis of a site that indicates that humans were present in the Mideast near the same time when they first started appearing in Europe. Thus, they most likely passed through a Levantine corridor on their way upward to Europe from Africa, which runs counter to recent doubts.

PNAS, 2015. DOI: 10.1073/pnas.1501529112 (About DOIs).