How Do Archaeologists Use Fossils

Using a geologist's magnifying drinking glass, Erin DiMaggio advisedly scans a slice of volcanic ash in search of tiny minerals that concur the central to determining the age of nearby fossils. Ramón Arrowsmith, Arizona State Academy

This is the quaternary in a v-part serial written past experts featured in the Smithsonian'southward new Hall of Fossils—Deep Time exhibition, at present on view at the National Museum of Natural History. The total serial can be found by visiting our Deep Time Special Report

"No fossil is cached with its birth document," wrote the renowned science editor Henry Gee in his 2000 treatise, In Search of Deep Time. While true, fossils are buried with plenty of clues that allow us to reconstruct their history. An array of absolute dating techniques has made it possible to plant the timescale of Earth's history, including the age and origin of life, the timing of mass extinctions and the record of human evolution.

In 2013, in Ethiopia'southward Distant region, our research team discovered a rare fossil jawbone belonging to our genus, Homo. To solve the mystery of when this man ancestor lived on Globe, we looked to nearby volcanic ash layers for answers. Using a geologist'south magnifying glass, we can carefully scan the ash in search of tiny minerals that are smaller than a unmarried sprinkle on a sugar cookie and that concord the key to determining the historic period of a fossil.

Working in this part of Ethiopia is quite the adventure. It is a region where xc degrees Fahrenheit seems absurd, grit is a given, h2o is non, and a normal daily commute includes racing ostriches and braking for camels as we forge paths through the desert. Only, this barren and hostile mural is one of the most important locations in the world for studying when and how early humans began walking upright, using tools and adapting to their changing environments.

How Do Scientists Date Fossils? — The Afar region has layered sedimentary rocks, containing thin white volcanic ash that we could use to date the fossil jawbone. Erin DiMaggio, Penn State University

Early, before we had more precise means to date fossils, geologists and paleontologists relied on relative dating methods. They looked at the position of sedimentary rocks to decide society. Imagine your laundry basket—the dirty clothes yous wore last weekend sit at the lesser, but today's rest on height of the pile. The concept for sedimentary rocks is the aforementioned. Older rocks are on the lesser, younger ones are on top. Researchers also used biostratigraphy, which is the study of how fossils announced, proliferate and disappear throughout the stone record, to institute relative ages. We still apply these relative dating methods today equally a first approach for dating fossils prior to assigning a numerical, or accented, age.

Tin we date actual fossils? Sometimes.

Scientists called geochronologists are experts in dating rocks and fossils, and can ofttimes date fossils younger than around l,000 years old using radiocarbon dating. This method has been used to provide dates for all kinds of interesting cloth like cave stone art and fossilized poop. Unfortunately, fossils like our jawbone, every bit well as the dinosaurs on view in the new "Fossil Hall—Deep Fourth dimension" exhibition at the Smithsonian's National Museum of Natural History, are only too quondam for radiocarbon dating. In these cases, nosotros have to rely on the rocks themselves. Nosotros engagement the rocks and by inference, we tin can date the fossils.

Rare fossil jawbone — In Afar, Ethiopia, a rare fossil jawbone belonging to our genus, *Homo* was discovered by our inquiry squad in 2013. To solve the mystery of when this human ancestor lived on Earth, we looked to volcanic ash layers for the answers. William H. Kimbel, Arizona State University

The offset large challenge is to find the right kind of rocks to collect for laboratory analysis. We are fortunate that the Afar region has volcanic ash horizons in the sedimentary rock layers. In fact, just beneath where the fossil jawbone was discovered, our squad found a new volcanic ash layer that we named the Gurumaha Tuff. The word gurumaha in the local language means milkshake—a reflection of our mindset in the 90 degree afternoon heat. In a sense, we can recollect of volcanic ash layers as buried stopwatches. When the volcano erupts the timer starts, and we use absolute dating techniques to tell the elapsed time.

Volcanic rocks typically contain naturally radioactive minerals—our sugar cookie sprinkles. We can date these minerals using techniques based on the radioactive decay of isotopes, which occurs at known rates. Measuring isotopes typically involves lasers and mass spectrometers and sometimes even nuclear reactors. We summate age using the disuse charge per unit and isotope measurements, which gives us the elapsed time on our stopwatch.

We successfully dated the Gurumaha Tuff to two.82 meg years old past dating the naturally radioactive mineral feldspar. Since the jawbone eroded from above the Gurumaha Tuff, it must be younger. We calculated the jawbone is between ii.fourscore and 2.75 one thousand thousand years onetime, making it the oldest known fossil of our genus Homo .

Geochronologists have an abundance of tools at their disposal, simply still, some rocks and fossils show difficult to date. Innovations to existing dating methods are eliminating these barriers. For instance, revisions to a method called electron spin resonance permit scientists to engagement rare fossils, like hominin teeth, because they can directly date the fossil without visibly damaging the specimen. In the Afar, scientists are attempting to date the actual layers from which the fossils erode, rather than relying on the presence of volcanic ash. This would more than accurately tell u.s.a. the age of fossil-bearing rocks, and open up new field sites for exploration that lack such layers.

Fossils bridge geologic time from hundreds to fifty-fifty billions of years and are discovered in many rock types and settings. Selecting a suitable dating technique is a critical step to obtaining a meaningful and accurate age.

Scientists have dated fossils found in South African caves to betwixt 236,000 to 335,000 years old using several different geochronometers, including optically stimulated luminescence, a tool that allows us to calculate the concluding time cave sediments were exposed to light. Another common method, uranium-lead dating, relies on the radioactive decay of uranium and can be used to engagement rocks containing the oldest known fossils on Earth—older than 3.5 billion years old! To put that in context, the historic period of the World is 4.54 billion years, just our species has just been around for approximately 300,000 years.

The age of the World and origin of life is nearly unfathomable, just the Smithsonian's new Deep Fourth dimension Hall is designed to help us grasp the enormity of Earth's history in low-cal of our nowadays-solar day impacts. The 18th-century geologist James Hutton recognized that geologic processes require long timescales, a concept cardinal to what nosotros mean by the term deep fourth dimension. The purpose of geochronology—dating rocks and fossils—is to weave together the dates we obtain to tell the extraordinary story of Earth'south deep time.