What's the world's oldest civilization?

Countless civilizations have risen and fallen over the millennia. But which one is the oldest on record?

About 30 years ago, this question seemed to have a straightforward answer. Around 4000 B.C., the earliest phase of the Sumerian culture arose as the oldest civilization in the Mesopotamia region, in what is now mostly Iraq. The Sumerians are named after the ancient city of Sumer, which was a few miles south of the modern city of Kut, in eastern Iraq. Archaeologists call the earliest Sumerian phase the Uruk period, after the equally ancient city of Uruk about 50 miles (80 kilometers) to the southwest, where many of the oldest Sumerian artifacts were found. 

But evidence uncovered in the past few decades indicates that the Sumerians have a few contenders, including ancient Egypt, for the title of "oldest civilization."

Related: What is the oldest-known archaeological site in the world? 

The definition of what makes a civilization is vague, but generally a culture has to achieve several hallmarks, notably urbanism — that is, cities — irrigation and writing; and the Sumerians had all three. After about 2000 B.C., the Sumerian civilization led directly to the Babylonian civilization in Mesopotamia, which is credited with discovering mathematical truths such as trigonometry and prime, square and cube numbers — concepts further developed by the ancient Greeks more than 1,000 years later. 

The Sumerians may have also invented religion by building towering temples called ziggurats in their cities and establishing priestly castes devoted to the ritual worship of specific deities, according to the American historian Samuel Noah Kramer. Which god was the mightiest in the vast Sumerian pantheon depended on the place and time: the sky god Anu, for example, was popular in early Uruk, while the storm god Enlil was worshiped in Sumer. Inanna — the "Queen of Heaven" — may have originally been a fertility goddess in Uruk; her worship spread to other Mesopotamian cities, where she was known as Ishtar, and may have influenced the goddesses of later civilizations, such as Astarte among the Hittites and the Greek Aphrodite. 

A story very like that of the Hebrew Bible's Noah, who built an ark stocked with animals to preserve his family during a great flood caused by divine wrath, is related in the Epic of Gilgamesh. Archaeologists think it was originally a Sumerian story from about 2150 B.C. — centuries before the Hebrew version was written.

Some scholars argue that other civilizations may be as old or even older than that of the Sumerians. "I would say that Egypt and Sumer were basically contemporary in their emergence," said Philip Jones, the associate curator and keeper of collections at the Babylonian section of Philadelphia's Penn Museum. 

Decades of war and unrest in Iraq have meant that archaeologists haven't been able to access many Mesopotamian sites, but Egyptologists have kept digging, Jones told Live Science. The result is that archaeologists in Egypt have now discovered writings just as early as the earliest writings from Sumer, which suggests the oldest phase of the ancient Egyptian civilization emerged at roughly the same time as the earliest phase of the Sumerian civilization: about 4000 B.C. 

Still another possibility is the Indus Valley civilization, which arose in parts of what is now Afghanistan, Pakistan and northwestern India, and dates from at least 3300 B.C., according to the earliest artifacts found there. But "we might find very early stuff in the Indus Valley," Jones said. "It wouldn't surprise me if we dug up something that was just as early." 

Jones suspects that early trade along the edges of the Indian Ocean helped these earliest civilizations — the Egyptian beside the Red Sea, the Sumerian at the north end of the Persian Gulf, and the Indus Valley civilization further east — develop from the pre-civilized people who lived there before them, by bringing them resources and ideas from further afield. 

"My gut feeling is that there was probably some trade networking going on the Indian Ocean," he said.

Originally published on Live Science.