The search for Earth’s most mysterious creatures is turning up extraordinary results

It’s easy to assume, as many people do, that our planet is well explored. In the last few centuries, humans have summited Earth’s highest peaks, dived its deepest ocean trenches, and trekked to the North and South poles, documenting the diversity of life along the way — the many birds, butterflies, fish, and other creatures with which we share our big planet.

Life on Earth is now largely known.

The more that scientists study the planet’s biodiversity, the more they realize how little of it we know. They estimate that for every species we’ve discovered, there are likely at least another nine or so that remain undiscovered or unidentified, meaning around 90 percent of life on Earth is unknown.

This doesn’t include the big stuff — the black bears and belugas and bald eagles, all of which have scientific names and descriptions published in academic journals. The unknown is made up of small organisms, such as insects, mites, and crustaceans. These species are the nuts and bolts of ecosystems: They produce soil, pollinate crops, and feed almost everything. And most of them have yet to be identified.

In just one fly family known as Cecidomyiidae, for example, scientists estimate there could be as many as 1.8 million species globally, and yet fewer than 7,000 have been described. This is especially remarkable given that the total number of described species across the entire animal kingdom is somewhere around 2 million.

Biologists describe animals like this as dark taxa, a term that refers to groups of organisms in which the bulk of species are undescribed or undiscovered. Some taxonomists have also called them biology’s dark matter.

“Most people think that life on Earth is described, and we have a good idea of how ecosystems are functioning,” said Emily Hartop, a fly researcher and taxonomist at the Norwegian University of Science and Technology, who studies dark taxa. “The reality is that for most species on Earth, we don’t know what they are, we don’t know where they are, we don’t know what they’re doing. They are unknown.”

Scientists who study dark taxa argue that lifting the shadow on these organisms is essential to our own survival. If we don’t know what constitutes our ecosystems, we risk killing off the key players that make them function — or failing to detect a potential threat, such as a disease-carrying insect that could set off the next global pandemic.

“The little things run the planet,” said Rudolf Meier, a researcher at Berlin’s Museum of Natural History and Humboldt University of Berlin who also studies dark taxa.

Hartop and some other researchers have dedicated their careers to exposing dark taxa — to making Earth’s unknown known. But filling these gaps is an enormous task and, until recently, considered impossible. The challenge comes down to process: How do you identify millions upon millions of species that are tiny, often look the same, and lack the traditional sort of charisma that funds expeditions?

Dark taxa biologists find hundreds of new species wherever they look

A little over a decade ago, when Hartop was living in Los Angeles, she and her colleagues set up bug traps in backyards across the city. They were mesh tents with openings, known as Malaise traps. Once flies buzz into them, they get stuck and navigate — rather unfortunately for them — into a vial of ethanol. The ethanol both kills and preserves the animals.

Over the course of just one year, the traps collected 99 species of scuttle flies, small insects in the family Phoridae that look, to my untrained eyes, a lot like fruit flies. Forty-three of those species were new to science and had never been described before.

When scientists look for dark taxa, they seem to find new species everywhere. Meier and his colleagues recently collected fungus gnats in Singapore, and their traps revealed 120 species. All but four or five were unknown to science. When researchers went looking for wasps in Costa Rica that parasitize other insects, they found 416 species. More than 400 of them hadn’t been described yet.

And the opportunity for discovery extends beyond the animal world. Scientists recently analyzed genetic codes from thousands of specimens of ectomycorrhizal fungi — a type of fungi that form symbiotic relationships with plant roots — and found that only around 20 percent of those codes matched known species.

Why have these organisms been overlooked for so long? One reason is that they’re typically small, often measuring less than 5 millimeters, Meier said. That makes them harder to notice — and less exciting by traditional standards.

“Funders are much more likely to give you money for birds and butterflies, because that’s something that a funder, who is not a biologist, finds much more relatable,” Meier told me. “If I want to get money for doing things on dark taxa, I first have to override these biases.”

But a far bigger obstacle is that these groups of life are extremely diverse. There are three species of elephants and eight species of bear. Meanwhile, there could be 1 million species of scuttle flies globally, Hartop said.

That creates a problem of scale. While trapping bugs in tents is easy, it’s much harder to identify them and demonstrate that they’re different from other species that have already been described. Until recently, it was nearly impossible.

We are in the Golden Age of discovery

For hundreds of years, scientists have largely categorized animals by their appearance. A toucan is obviously different from a robin, which is obviously different from a hummingbird. Scientists use these distinctions in form to separate animals by species, typically defined as organisms that reproduce with each other but not with other animal groups.

The study of form, known as morphology, has been used to categorize small things, too, such as moths and butterflies. But for some animal groups — scuttle flies, mites, and nematodes, for example — this approach is inadequate. While distinguishing these animals by appearance is often possible, it typically requires an enormous amount of time and expertise; scientists literally have to look at them one by one through a microscope. Plus, looks can be deceiving: A bunch of, say, black-and-blue butterflies might appear identical but come from different genetic lineages that make them distinct species.

That’s why a technology called DNA sequencing has been such a game-changer. In the 1970s, scientists figured out how to sequence part of an organism’s DNA, producing a string of letters that corresponds to its genes. They later discovered that they could use just a small snippet of that sequence to tell one species apart from another. In 2003, a Canadian biologist named Paul Hebert dubbed those snippets “barcodes” because they serve as unique species IDs, akin to barcodes on cereal boxes in the grocery store.

Over time, scientists sequenced animals and uploaded their barcodes to databases, helping organize and reorganize the animal kingdom. All the while, the technology evolved. DNA sequencing is now so advanced that taxonomists — those who classify life — can barcode thousands of specimens at one time.

It’s this approach that’s helping illuminate dark taxa: Researchers can collect scores of specimens from the field, sequence portions of their DNA, and then upload those bits of code to an existing database to see if they match known species. If not, they might represent something new.

Even with modern DNA sequencing, identifying unknown life is, to be clear, still very hard. A big issue is that there aren’t barcodes for most species that scientists have already described. Museums might have physical specimens — dead moths or beetles in a drawer in their basement — that lack genetic data in online databases. So just sequencing a discovery is usually not enough to prove that something is new to science.

When scientists are confident that they’ve found something new, they’ll face additional challenges if they want to formally describe the animal and give it a scientific name. That typically requires multiple lines of evidence and a description published in a scientific journal. Doing that for dark taxa — which, again, have hundreds of thousands of unknown species — would be incredibly time-consuming. (The world of taxonomy is full of drama about the species-naming process and how much evidence scientists should be required to provide. There’s also a debate about whether formally naming species actually matters if they already have unique DNA sequences that identify them.)

Nonetheless, modern DNA sequencing has massively sped up the process for discovering and identifying life. It’s pretty extraordinary: Even though we’ve known about the most visible species around us for hundreds of years, only now are we in the Golden Age of species discovery.

“It’s unbelievable,” said Hebert, a professor at the University of Guelph in Ontario who oversees the Center for Biodiversity Genomics, a DNA-barcoding research center. “This is the age of bio-discovery.”

Can we describe all life on Earth?

That’s the goal. While there are no reliable estimates for the total number of species on Earth, it’s likely in the tens of millions. And again, only around 2 million are formally described, Hebert said.

Before modern sequencing became a reality, identifying all life on Earth would have taken hundreds, if not thousands, of years and likely would have cost trillions of dollars. Now, some scientists are confident that they can do it in a matter of decades or even years.

In 2005, Hebert launched a project with his colleague Sujeevan Ratnasingham that is essentially trying to collect DNA data for every animal on Earth. So far, the project — known as Barcode of Life — has sequences for roughly 1.5 million species, Hebert said, though many of those are not formally described. To barcode the rest would require no more than $1 billion, he told me confidently. That money would help fund expeditions and DNA sequencing around the world.

“We want barcode records for every species,” Hebert said. “If I can persuade the world to support this with about $1 billion, which is trivial, we can complete the inventory of animal life by 2040 — I am certain.”

Hebert and other taxonomists imagine a world in which all species are known and can thus be tracked. Just as we monitor the weather for looming disasters, complete inventories of animal life could allow scientists to monitor biodiversity — both the obvious and obscure stuff — to see how our ecosystems are changing and what that means for us. Are ocean food chains we rely on shrinking? Are the insect larvae that make our soils fertile in decline? Is a pathogen on the loose?

But there’s also a more noble reason to discover life, he says. “This is the planet we live on,” Hebert said. “We really should understand the organisms that we share it with.”

And if you’ve got a billion dollars lying around, you can apparently help.

“For a billionaire, it’s a no-brainer,” Hebert said. “That’s a legacy for that person. You only get to do it once: discover life on our planet.”