Predicting the past with the periodic table

Predicting the future is the pinnacle of what science can do. It’s impressive enough for a scientist to look at existing data and compose a theory explaining it. It’s even more impressive for a scientist to predict what data will look like before they are collected. The periodic table is central to chemistry precisely because it has both explanatory and predictive power.

From the time the periodic table was first assembled, it has helped predict future chemical data. But the periodic table applies to more than just chemistry, and it has predicted future data in other areas as well. Here is a case in which the periodic table helped predict biological data—the biochemical timing of certain genes—before those genes were found and sequenced.

Instead of just predicting the future, the periodic table predicted the past—or rather, it predicted future data about the past.

The predictive power of the periodic table was evident from the beginning. Exactly 150 years ago, Dmitri Mendeleev wrote chemical properties on 63 cards and slid them around in history’s most productive version of Solitaire. Once Mendeleev recognized how these cards stack up in groups of eight, he established the basic order of the periodic table. This order revealed four gaps in the table, which Mendeleev predicted would be discovered in the future as elements with particular atomic weights and other chemical properties. Mendeleev’s table predicted scandium, gallium, technetium, and germanium before any of these were isolated in a lab.

Before Mendeleev, other chemists had organized other periodic tables, but these tables aren’t used anymore because they were not predictive.

Some of these predictions related to Mendeleev’s periodic table had implications beyond chemistry, extending to biology and geology. One such prediction began 66 years ago with a pair of inorganic chemists, who found that the periodic table helps predict how transition metals work. They measured how tightly six transition metal ions bind to non-metals. When they arranged the metals in periodic table order, no matter which non-metals they used, they found a graph with a consistent upside-down “V”, peaking at copper (see the graph on the left).

This means that copper, on the right side of the transition metals, sticks to pretty much anything, while manganese, at the far left, sticks to things far more weakly. This simple trend proved powerful in the field of inorganic chemistry. This graph is taught to chemistry majors today, named after the two chemists, as the “Irving-Williams series.”

One of the chemists was Robert J.P. Williams (1926-2015), who was only an undergraduate at the time. He went on to write a foundational inorganic chemistry textbook and made discoveries ranging from how the iron in hemoglobin works to how mitochondria create cellular energy. Williams liked to apply chemical reasoning to biological contexts, with considerable success.

During the final decades of his life, Williams expanded his scope and applied chemical reasoning to the four-billion-year history of life on this planet. He made a bold prediction: life evolved according to a predictable chemical sequence, which itself followed the order of the periodic table.

To reach this conclusion, Williams started with the series that carries his name. He observed that cells are constrained to follow the Irving-Williams series when they pump metal ions in and out to maintain particular metal concentrations inside the cytoplasm. Concentrations of sticky ions like copper must be kept low lest they stick to the wrong thing, while less sticky ions like manganese can be kept at higher concentrations. This results in a graph of cytoplasmic transition metal concentrations being the exact inverse of the Irving-Williams series: when binding is tight, concentrations are low, and vice versa (see the graph on the right). This general rule applies to life at all places – and at all times.

Williams combined three observations to make a prediction:

1. Chemical observation: Transition metal stickiness follows the periodic table order.
2. Geological observation: Oxygen increased in the air over billions of years and oxidized the entire planet, including some transition metals.
3. Biological observation: Cells need to use metals that are dissolved and free in solution, not stuck in a big chunk of rock.

By following the chemical rules of how oxygen would react and how metals combine and dissolve, Williams argued that life on the ancient earth could only use elements on the left side of the series (manganese, iron, cobalt, and nickel) while more recent, complex life use more elements on the right side of the series (copper and zinc).

The record of which metals life used, and when the metals were used, is written in thousands of genes found in millions of organisms. Williams predicted what future gene sequencing would reveal before DNA sequencing technology came of age, revealing what genes were found in organisms across the tree of life and which metals they used. By the middle of the first decade of the 21^st century, enough data was in, allowing three separate groups to confirm Williams’ theory: indeed, ancient organisms emphasized the left side of the series and new organisms the right.

Because the periodic table is universal, Williams’s ideas are also universal and continue to apply to data yet to be collected. Other water-based planets should follow the Irving-Williams series and alien life should use the same metals for the same biochemical jobs (if confounding astrobiological factors don’t confound). What’s especially exciting is that we are moving into an age of discovery in this particular area. Exactly like DNA sequencing gave data about natural history a decade ago, over the next decade, astrobiology should give data about exoplanets’ atmospheric chemistry that may start to reveal how Williams did.

Williams looked into life’s history and saw the pattern of the periodic table behind it. He looked past the chaos of biochemistry and geology to the orderly columns of the periodic table scaffolding and constraining the possibilities, setting chemical rules and sequences for the planet to follow. Williams showed where the biological data would go, and later, the biological data followed his theory.

Mendeleev could not have anticipated this when he was shuffling cards around 150 years ago, but the order of elements that he discovered set a chemical order for both mineral and biological evolution on this planet. Who knows? It may have done so on others as well.