About 50,000 years ago, modern humans left Africa and began occupying the rest of the world. The common thought is that a sudden growth in population caused the so-called “human revolution,” which gave birth to language, art, and culture as we know it today. Now, based on something that’s not obviously related to human culture—the size of shellfish fossils—researchers have challenged that model.

Artifacts from two sites in South Africa, Still Bay and Howieson’s Poort, have convinced archaeologists that the period between 85,000 to 65,000 years ago was when the “human revolution” began. Humans from that time made jewelry from perforated shells and used objects as symbols. They made better tools than they had ever before. Some of these tools, made from ostrich eggshells, were even capable of slicing fruit.

It has been thought that this period also saw a sudden explosion in population growth. Now, Richard Klein from Stanford University and Teresa Steele from the University of California at Davis argue that archaeologists and anthropologists have got it wrong. In a study published in the Proceedings of the National Academy of Sciences, they say there is evidence to suggest that population did not actually explode during this period.

Klein and Steele’s evidence comes from the size of shellfish fossils from that period. Higher predation of shellfish forces their shells to become smaller, and there is no evidence of shellfish shrinkage during this time.

According to Robert Foley, an expert on the origins of modern humans at Cambridge University, the idea is not as far-fetched as it sounds. “Other things remaining equal, there is a strong relationship between shell size and human predation,” he said. This correlation can be seen in shellfish around the world today. Coasts where shellfish are collected en masse see animals with smaller shells than those where shellfish aren’t collected.

The evidence for Klein and Steele’s claim comes from the South Africa, which seems to be the only place that has well-preserved shellfish fossils from that period. They find that the shell size during the Middle Stone Age (MSA, about 200,000 years ago) and Later Stone Age (LSA, about 50,000 years ago) were not much different. Thus, they say, this period must not have seen a sudden population explosion, as many argue.

But Martin Zeigler, climate geologist at the Swiss Federal Institute of Technology in Zurich, is not entirely convinced. “Environmental factors could affect shellfish size on the timescales that are studied here,” he said. And he is right. Many factors are involved in determining shell size, such as water temperature, salinity, nutrient availability, and species population.

Klein and Steele argue that we will never have enough information on how much these factors played a role. But with studies on contemporary shellfish showing the same trend, it's hard to argue against the hypothesis without some evidence showing that it’s wrong.

Foley has respect for Klein’s work. He said, “They do tackle limitations in their study by pointing out that rising sea-level has removed the relevant sites from the recent LSA period of about 10,000 years ago. While one has to be cautious about the overall implications of their analysis for the evolution of modern humans, it is not very likely that rising population could ever be a complete explanation.”

Klein and Steele believe that the innovations that happened in Still Bay and Howieson’s Poort at the time cannot be explained by an enhancement in human ability to survive and reproduce. Instead, they argue that other reasons, such as climate fluctuations and genetic changes, may explain what caused the human revolution.

PNAS, June 2013. DOI: 10.1073/pnas.1304750110 (About DOIs)

This article was first published at The Conversation.