A rebuilt enzyme from Earth’s deep past has preserved the same chemical fingerprint seen in ancient rocks, confirming that a key signal of life has remained stable for billions of years.
That continuity reshapes how scientists read the planet’s earliest history and strengthens one of the few tools available for recognizing life beyond Earth.
Ancient enzymes from early life
Inside living microbes, the finding takes shape in the behavior of nitrogenase, the enzyme that allows organisms to turn atmospheric nitrogen into biological fuel.
By reconstructing ancient versions of that enzyme and watching them function in cells, Betül Kaçar and colleagues at the University of Wisconsin-Madison (UW-Madison) demonstrated that the isotopic imprint tied to nitrogen fixation matches the record locked into primordial sediments.
The result holds even as the enzyme’s genetic sequence and performance changed across deep time, answering a long-standing uncertainty about whether modern chemistry could mislead interpretations of early life.
That constraint sets clear boundaries on what nitrogen isotopes can and cannot reveal, opening the door to a closer look at why this molecular signal endured while other traits evolved.
Why nitrogen is essential for life
Life needs nitrogen to build proteins and genetic material, but most organisms cannot use the gas floating overhead.
Some microbes solve that by nitrogen fixation, changing nitrogen gas into ammonia that cells can absorb.
Nitrogenase does that conversion, and crops depend on bacterial partners that live around roots or drift through water.
When nitrogenase fails, ecosystems slow down, which explains why a clue about its ancient behavior carries real weight.
Rocks preserve signs of life
In some 3.2-billion-year-old rocks, nitrogen isotopes, forms of an element with different neutron counts, hint at early biology.
Scientists compare the balance of heavy and light nitrogen in those rocks, because biology usually favors one form over another.
That pattern acts as a biosignature, a measurable clue that living things were present, even when fossils never formed.
But the whole approach depends on one bet: ancient enzymes had to leave the same isotope pattern as modern ones.
Old genes inside new cells
Instead of waiting for rare fossils, the researchers wrote ancient genes from scratch and installed them in a modern bacterium.
That approach uses synthetic biology, building and rewriting genes to give cells new jobs, to recreate proteins that no longer exist.
The team chose a microbe called Azotobacter vinelandii because it already fixes nitrogen and tolerates oxygen better than many species.
By swapping its own enzyme parts for reconstructed ones, the bacteria had to rely on those ancient instructions to grow.
Chemical life stayed stable
After the engineered microbes fixed nitrogen, the team measured how much heavy nitrogen ended up in cell material.
Across enzymes separated by more than two billion years of evolution, the chemical signal stayed tightly clustered rather than drifting over time.
That consistency matches what scientists see in living nitrogen-fixing microbes today, reinforcing confidence that the same signal in ancient rocks reflects real biological activity rather than later distortion.
The team could finally treat nitrogen isotope clues as stable over deep time, rather than a moving target.
Enzyme changed life signal
Resurrected enzymes did not behave exactly like modern ones, even though the microbes still survived on nitrogen alone.
Some ancient versions ran slower in lab tests, which showed that the protein machinery had changed while keeping core chemistry.
Yet the isotope pattern stayed tied to the same step in the reaction, so mutations did not rewrite the signal.
That split, changing speed but constant signature, hints that evolution tuned performance without touching the part geologists measure.
Reading early Earth anew
Because the signature stayed stable, geologists can read nitrogen isotopes with more confidence when they study the oldest sediments.
That matters most before the Great Oxidation Event, a long rise of oxygen in air, when microbes ruled without it.
Back then, carbon dioxide and methane filled more of the sky, and nitrogen-fixing microbes helped keep ecosystems running.
If those microbes always left the same isotope mark, researchers can separate true biological signals from later chemistry more cleanly.
Searching life elsewhere
NASA cares about stable biosignatures because rovers and landers need chemical targets that survive billions of years of change.
Kaçar’s lab at UW-Madison sits inside a NASA-funded group studying life across deep time to guide future searches.
Earth formed about 4.54 billion years ago, and the U.S. Geological Survey shows how scientists pinned down that number.
“As astrobiologists, we rely on understanding our planet to understand life in the universe.” Kaçar says.
Enzymes and life on Earth
Reconstructed enzymes came from one branch of nitrogenase history, so they cannot represent every ancient microbe.
Lab conditions also stay controlled, which means temperature, ocean chemistry, and pressure changes from early Earth still need tests.
Kaçar’s team wants to learn why isotope control stayed put even while other parts of the enzyme kept evolving.
Answering that could sharpen how NASA missions judge odd chemistry on Mars, while also warning against over-reading any single signal.
The rebuilt enzyme work links lab biology to rock data, showing that one core reaction kept a consistent stamp.
Future experiments can extend this approach to other enzymes, but future teams still need many clues before calling anything alive.
The study is published in Nature Communications.
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