http://fivethirtyeight.com/features/how-do-we-know-when-a-hunk-of-rock-is-actually-a-stone-tool/

The first time archaeologist John Shea looked at what might be the oldest stone tools ever found, he almost blew them off. “Are you kidding me?” he remembers asking Sonia Harmand, his colleague at Stony Brook University who found the tools in 2011 along the shores of Kenya’s Lake Turkana, at a site now known as Lomekwi 3. Harmand’s analysis suggested that the tools were 3.3 million years old — 700,000 years older than the previously known “oldest” tools. And they were huge. The mean weight of the pointy flakes — the cutting tools that are “flaked” off larger rock cores — was 2 pounds. In comparison, the next-largest group of ancient hominin tool flakes have a mean weight of 0.06 pounds. It seemed like a lot for the hands of a primate that was probably half the size of the average modern human. Maybe, this time, a rock was just a rock. But then Shea looked at the Lomekwi tools more closely, and he saw multiple fractures, all running in the same direction — a telltale sign that the flakes weren’t just the lucky product of one rock bumping into another as it tumbled off a cliff or rolled through a stream. Something had pounded one rock against another in the same place over and over and over … until a sharp piece broke off.

“Harmand’s analysis suggested that the tools were 3.3 million years old ‘’

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“suggested” How do we know this is right?

I was going to make an off the cuff comment and state “that it’s just a yonie”, a piece of slang from my youth for a stone.

Looked up the spelling and was reasonably surprised to find what the search ‘yonie + stone’ results in.

Here is a 28,000 years old Dildo
http://www.theregister.co.uk/2005/07/27/ancient_phallus/

tauto said:

“Harmand’s analysis suggested that the tools were 3.3 million years old ‘’

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“suggested” How do we know this is right?

The normal method is to find something datable in the same stratographic layer as the stone. 3.3 million years is particularly tough because it’s too old for reliable carbon 14 dating and too young for most other radiogenic methods.

Less accurate methods include fluorine dating, and the thickness of the weathered layer on the stone itself.

Looking up the technical; article “The chronological context derives from correlation with the Lomekwi Member of the Nachukui Formation and radiometrically
dated tuffs within it, as well as from magnetostratigraphy of the site and estimated sedimentation rates.” Tuffs are volcanic, which makes them easy to date.

Checking article – “stone tools made by capuchin monkeys”, interesting. “Last month, Oxford archaeologist Tomos Proffitt published a paper that documents wild capuchin monkeys in Brazil cheerfully screwing with a whole scientific field by intentionally picking up rocks and banging them against other rocks over and over and over in the same place … until sharp pieces broke off.2 There’s no evidence that the monkeys were trying to make tools, and they generally ignored the flakes. Their behavior could be an aggressive display or possibly a means of getting at lichens or trace minerals in quartz dust. Either way, the accidental tools they produce from intentional rock-banging are virtually indistinguishable from what you might find in 3-million-year-old East African soil.”

On original topic, (not the capuchin monkeys) see:
Technical article from Nature

Previous earliest tool use:
“The earliest known artefacts from the sites of Gona (2.6 Ma), Hadar (2.36 Ma), and Omo (2.34 Ma) in Ethiopia, and Lokalalei 2C (2.34 Ma) in Kenya, demonstrate that these hominin knappers already had considerable abilities in terms of planning depth, manual dexterity and raw material selectivity. Cut-marked bones from Dikika, Ethiopia, dated at 3.39 Ma, has added to speculation on pre-2.6-Ma hominin stone tool use.”