16: Life Before Fossils

For a dozen episodes, I’ve told you the tale of the Earth’s formation, the Moon’s formation, the earliest core, crust, and oceans. And now for something completely different. Many planets and moons have cores and rocky crusts, quite a few have water or ice, but as of 2022, Earth is the only place we know that has life. One of the biggest mysteries in science, and one of the fundamental questions of humanity is: where did life on Earth come from?

 Hidden within that question is an even bigger question that must be asked first: What is life? If we can’t define what life is, then how do we know it when we see it? To reference Douglas Adams, it’s like knowing the answer to life, the universe, and everything without knowing what the question is.

 And yet, ask ten scientists what the definition of life is, and you’ll get twenty answers. I could spend a dozen episodes in a row just defining life. Long time listeners of the show know that’s not an idle threat.

At this point in our series, we’re sitting in the Hadean after the Moon’s formation, 4.4 to 4.0 billion years ago, late January in the Earth Calendar. The first definitive fossils, the ones everyone agrees on, are a billion years younger, late March on the Calendar. We won’t see this fossil pond scum for another two seasons of this show. So instead of tackling the origins of life all at once, we’re going to divide and conquer this question into a series of bite-sized topics.

One thing scientists do agree on is that life on Earth began before the earliest fossils. Proving it is a completely different story, and there are many tales to tell over the next billion years: discoveries, debates, and dead ends. Today, we take one final look at the Jack Hills zircons to see if there’s anything in Earth’s oldest crystals that tells us about the origins of life.

Part 1: Carbon Copies

Every organism on Earth, from bacteria to Bruce Springsteen, uses carbon as a fundamental building block. 20% of your body is made of carbon atoms, and it’s the fourth most common element in the universe, after hydrogen, helium, and oxygen. It’s easy to take a carbon-centric view of the world. After all, forests, fields, and every living thing is made of it, so it must be common on Earth, right?

As I mentioned with water, carbon isn’t rare, but it’s not as abundant as you’d think. Only a fraction of a percent of Earth’s crust is made of carbon. In contrast, silicon and oxygen are key ingredients of nearly every mineral we’ve met on the show, making up more three quarters of the Earth’s crust.

But what carbon lacks in abundance, it makes up in flexibility.

Each element of the periodic table is like a Lego brick- they can usually attach to other elements, but some fit together better than others. Many atoms can only assemble in certain patterns. They’re fussy and must be arranged just so. These inflexible rules and arrangements make the geometric patterns of many minerals: there’s a reason that table salt always forms cubes.

In contrast, carbon is the closest thing to a universal Lego brick- it can easily interact with many elements across the table, forming strong but flexible bonds with different atoms. Unlike crystalline minerals, carbon’s flexibility allows it to form long, strange, irregular molecules that look more like modern art than well-behaved geometry. You can see this plasticity when you pinch your own skin, bend a plastic spoon, or in the double-helix of DNA.

Scientists such as Carl Sagan noted that it’s possible for other elements like silicon to create complex patterns, and argued against “carbon chauvinism” when looking for alien life. However, the same scientists also note that carbon is still the best candidate to form life which grows, consumes, and evolves, especially for conditions on Earth. So for this program, we shall remain carbon-focused. There’s a reason that table salt doesn’t form a double-helix.

On the other hand, carbon can also make geometric crystals like diamonds and graphite, as if poking fun of its’ stiff, inflexible neighbors. But carbon minerals only form under intense heat and pressure: you don’t see diamond trees or graphite grass on Earth, though that would be an interesting planet.

In Precambrian rocks, carbon usually survives in four forms: diamonds, graphite, cooked organic carbon, and limestone. If your goal is to find the oldest traces of life on Earth, these materials are a good place to start. Speaking of which, let’s see which flavors of carbon are preserved inside our old mineral friends, the Jack Hills zircons.

Part 2: Diamonds in the Rough

Since Episode 10, we’ve been exploring the chemical toolkit inside zircon crystals. Whenever a new zircon is discovered, we can turn to these elements to tell us a story. Uranium for time, hafnium for the crust, and oxygen for water. Today we look at the final tools in the set. Unlike all the others, these tools aren’t elements: they’re other minerals.

That’s right, if you look carefully inside a zircon crystal, you can often find bits of other crystals floating inside. How is this possible?

Diamonds embedded in a 4.25 billion-year-old zircon (Menneken et al., 2007)

Zircons are born inside molten rock, usually in magma chambers nestled deep within the crust. As the magma cools, zircons begin to crystallize and grow like snowflakes. But this process is not neat and clean. Remember, there are many other minerals forming at the same time, and the pool quickly becomes crowded. Imagine that you’re a zircon crystal floating in the magma pool, slowly growing, minding your own business. You feel a bump on your side- a tiny crystal of quartz has pushed up beside you. If the quartz sticks around long enough, you will slowly grow around it, swallowing it up like an old tree growing into a fence. These tiny hitchhikers are called inclusions, because they’re included inside larger minerals.

In 2007, six years after the papers describing the oldest zircons and the first water on Earth, another paper was published in the journal Nature. This new paper, with Martina Menneken as the lead author, was the first to describe diamonds in the Jack Hills zircons, the oldest diamonds on Earth.

Even if you’re not a geologist, you might know that diamonds need intense heat and pressure to form. The most common diamond recipe is to bury carbon deep within the mantle. As we learned in Episode 13, the Jack Hills zircons formed in the crust, so that doesn’t work. Option 2: a deep volcanic eruption flash-bakes carbon at supersonic speeds. Such events   are well-known from South Africa, but they look nothing like the Jack Hills in Western Australia. Finally, diamonds can form during meteor impacts, smashing carbon into crystals. Again, the Jack Hills samples look completely different.

Menneken and company concluded that the diamonds formed under a very thick crust. It was better than the others, but it also had a big problem. Under such intense pressure, other minerals should also form, such as tortured forms of quartz. These minerals were notably absent, and the debates continued.

Light and heavy “isotopes” of dogs.

One year later, the same group published another Nature paper, led by Alexander Nemchin. This new paper tackled the mystery of the Jack Hills diamonds using a different angle: carbon isotopes. Last episode, we learned that isotopes are atoms of the same element with different masses, just like Chihuahuas and Great Danes are both dogs with different masses. We learned that oxygen has light and heavy isotopes that move from water into minerals- this chemical dance is how we know about Earth’s earliest oceans.

Carbon works in a similar way. Living things love to gobble up light carbon isotopes- this is an important theme we’ll see throughout the entire podcast. Here’s how I remember this idea: if I asked you to carry a Chihuahua or a Great Dane all day, I think you’d choose the Chihuahua. Life does the same thing with carbon- it chooses light over heavy every time. If you’re a scientist and you find light carbon isotopes in your rocks, it’s possible that those atoms used to be trapped inside a living thing. Possible.

When Nemchin and company looked at the Jack Hills diamonds, they saw very light carbon. The paper was honest and explained several ways to make light carbon, but one of those recipes does involve living things.

To recap: in 2007, researchers found tiny diamonds in the Jack Hills zircons. These diamonds contained light carbon atoms, possibly recycled from living organisms. So, why aren’t these diamonds the oldest evidence for life on Earth? What gives?

As the years passed, other researchers examined hundreds of Jack Hills zircons and found no diamonds at all. Questions kept piling up. The answer finally came seven years later. It turns out, the diamonds were made by living creatures: humans.

Cross-section of a Jack Hills zircon showing abrasion with diamond fragments (*), from Dobrzhinetskaya et al., 2014. The view is 3 microns wide.

To be crystal clear, this was not a scam, it was an honest mistake. It turns out, to prepare a zircon crystal for analysis, you need to polish it down. As we’ve mentioned time and again, zircons are some of the toughest crystals on the planet. If you need a good polish, diamond grit is a great solution. In 2014, Menneken, Nemchin, and their team generously gave their samples to a group at UC Riverside led by Larissa Dobrzhinetskaya, who discovered that the diamonds were not inclusions from Hadean magma, but were fragments of polishing grit embedded within the zircons.

That might have been a long story just to say it was all human error, but there is a point to it. First, my goal is not to rag on the discoverers. The team could have refused to give away their samples, but instead they were generous, and this led to the truth. Working with other scientists can be frustrating to say the least. But the example I just showed proves why honest communication and collaboration are crucial to the progress of science. We don’t call ourselves “the scientific community” for nothing.

Part 3: The Hunt Continues 

In 2015, the year after the Jack Hills diamonds were revealed to be modern, the next chapter in the search for Hadean life began. This time, researchers were hunting for another carbon mineral: graphite.

Graphite is a shiny dark gray mineral that leaves black marks on everything it touches. You can see graphite every day inside pencils. You’ll still hear people call graphite “pencil lead”, because it was easy to confuse graphite with lead minerals inside an old mine. Fortunately for your health, there’s no real lead inside graphite, so sucking on a pencil tip will not give you lead poisoning.

On this podcast, will see graphite far more frequently than diamonds, especially in the search for early life. Graphite needs heat and pressure to form, but far less than diamond, so it’s more common. However, when Elizabeth Bell at UCLA went searching for graphite inside 10,000 Jack Hills zircons, she only found one sample.

We’ve just seen that zircon research can be tricky, so let’s quickly follow the checklist Bell and her colleagues made.

1: Was this graphite Hadean? Yes

The graphite was sealed tight by the surrounding zircon crystal, like a time capsule. Apart from human polishing, minerals can sneak inside a zircon long, long after the magma chamber cooled. Zircons are tough, but they can start to crack over four billion years. This breach in the gates can be infiltrated, leading to minerals that tell us nothing about the Hadean world. It’s like asking the current owners of an old house how they built it: they weren’t around back then.

2: Did the graphite have light or heavy carbon?

Carbon isotopes from early minerals. Lighter isotopes are lower on the chart, such as organic carbon and Jack Hills graphite (∆), from Bell et al., 2015

Remember, living things prefer carrying around light carbon, and those patterns can be inherited by minerals like graphite. Bell discovered that the Jack Hills graphite was very light, as light as carbon atoms inside living organisms like bacteria.

So, case closed! Break out the champagne, this is the earliest evidence for life on Earth!

Well, as of 2022, the scientific jury is still deliberating. It turns out there are many ways to make light carbon. Not all of them need living things. To be fair, Bell and her colleagues addressed these problems head-on, providing a laundry list of alternative scenarios. I’m not going to go through all five here, but most of them do have a common theme.

Until now, we’ve talked about carbon inside living things or minerals. But carbon is also present in gases like carbon dioxide, carbon monoxide, and methane. When we think of gases on Earth, we usually look up to the skies, but there are many gases trapped below us in Earth’s crust. Just like minerals, heat and pressure change these gases into different forms. If you cook carbon monoxide just right, you can turn it into a carbon-rich liquid. In fact, this recipe is used by humans today to make synthetic fuel.

Unfortunately for geologists, the same temperatures and pressures that form liquid carbon are the same as in cooked ancient rocks. Even worse, the carbon isotopes of this fluid can sometimes look identical to living things. It’s like looking at a picture of a celebrity standing beside a well-made wax figurine. Sometimes it’s hard to tell which one is the living thing.

We will dive deeper into this problem in future episodes, it will be the bane of our existence for millions of years to come. For now, here’s what we can say:

Life probably existed on Earth in the Hadean, between 4.4 and 4.0 billion years ago. There was water, the temperatures were right, and there were energy sources to munch on. Just like today, ancient life was probably made of light carbon atoms. This carbon would get buried and recycled into minerals like diamonds and graphite.

Scientists are still searching for light carbon minerals, trapped inside the Jack Hills zircons. Finding these crystals is just the first step, for there are many other ways to shape carbon on Earth. Perhaps Dr. Bell’s graphite is the first evidence of life on Earth, or perhaps we need to look elsewhere. For now, the great hunt continues.

Next episode, we finally move beyond the Jack Hills, forward in time into Earth’s missing years, 4.4 to 4.0 billion years ago. We’ll continue discussing the origins of life, including what our very first ancestor might have looked like.

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Thank you for listening to Bedrock, a part of Be Giants Media.

If you like what you’ve heard today, please take a second to rate our show wherever you tune in- just a simple click of the stars, no words needed unless you feel like it. If just one person rates the show every week or tells a friend, that makes us more visible to other curious folks. It always makes my day, and that one person could be you. You can drop me a line at bedrock.mailbox@gmail.com. See you next time!

Images:

Carbon atom: https://commons.wikimedia.org/wiki/File:Carbon_atom_(Bohr_model).png

DNA: https://commons.wikimedia.org/wiki/File:DNA_Structure%2BKey%2BLabelled.pn_NoBB.png

Diamonds in Jack Hills Zircon (original paper): Menneken et al., 2007

Dogs: https://commons.wikimedia.org/wiki/File:Big_and_little_dog_1.jpg

Diamonds in Jack Hills Zircon (cross-section): Dobrzhinetskaya et al., 2014

Graphite: https://commons.wikimedia.org/wiki/File:Graphite-233436.jpg

Early carbon isotopes: Bell et al., 2015

Music:

Voliere (Aviary) from Carnival of the Animals, Camille Saint-Saens: https://commons.wikimedia.org/wiki/File:Saint-Saens_-_The_Carnival_of_the_Animals_-_10_Volière.ogg

A Look Inside by Doo Dah Music

The Goldberg Variations: No. 3 by Johann Sebastian Bach, performed by Kimiko Ishizaka https://commons.wikimedia.org/wiki/File:Goldberg_Variations_04_Variatio_3_a_1_Clav._Canone_all%27Unisuono.ogg

The Goldberg Variations: No. 5 by Johann Sebastian Bach, performed by Kimiko Ishizaka

https://commons.wikimedia.org/wiki/File:Goldberg_Variations_06_Variatio_5_a_1_ovvero_2_Clav.ogg

Aquarium from Carnival of the Animals, Camile Saint-Saens:

https://commons.wikimedia.org/wiki/File:Saint-Saens_-_The_Carnival_of_the_Animals_-_07_Aquarium.ogg

Rondeau from Abdelazer by Henry Purcell, performed by Adieu Adieu

TV Mambo by Daniel Belardinelli

Leyenda (Asturias) by Isaac Albéniz, performed by Eric B. Davis

Bicycle Bell by TeWeBs https://commons.wikimedia.org/wiki/File:Ding_Dong_Bicycle_Bell_A.ogg

Their Arrival by Emmett Cooke

Seven Days of Flying by Remember the Future