You’ve probably heard before that Earth has witnessed five mass extinctions. It’s a number taken straight from the fossil record, but that’s not the whole picture, not even close.
 Evidence from ancient rocks reveals the oldest die-off in our planet’s history most likely occurred among single-celled organisms more than 2 billion years ago. A recent analysis now suggests that the deadly event was far more significant than any known loss of life that came after, even the dinosaur extinction. “This shows that even when biology on Earth is comprised entirely of microbes, you can still have what could be considered an enormous die-off event that otherwise is not recorded in the fossil record,” says geologist Malcolm Hodgskiss from Stanford University.
 Researchers describe this time as a tale of ‘feast and famine’, long before the rise of complex life and far beyond the fossil record. Starting 2.4 billion years ago, oxygen in Earth’s atmosphere was scarce, primarily trapped in minerals or locked up in the water. Then, cyanobacteria arrived on the scene, and these tiny marine microorganisms could photosynthesise, using sunlight to ‘breathe’ oxygen into the air.
 The Great Oxidation Event (GOE) was a feasting time as life flourished, diversified, and spread. And then, for some reason, it all came to an abrupt and catastrophic end. To figure out why, scientists at Stanford turned to a mineral called barite, found on the Belcher Islands of subarctic Canada. The landscape here has survived for billions of years, which means it holds a long record of oxygen in our planet’s atmosphere.
 Measuring oxygen, sulfur and barium isotopes found in these rocks, the authors found harmful oxygen isotopes occurring shortly after the GOE, some 2.05 billion years ago. This would have required a rapid reduction in the primary production of bacteria, dropping by more than fivefold and potentially by as much as 200 fold. Such a massive collapse, the authors argue, was probably triggered by a lack of critical nutrients, such as phosphorus, which is an essential factor in determining the distribution and development of cyanobacteria.
 “A large decrease in nutrients likely brought about this drop supplied to the biosphere that, in turn, marked the conclusion of the GOE and led to the subsequent 1-billion-year interval characterised by markedly low and stable gross primary production compared with the modern Earth,” the authors write.
 The findings support the ‘oxygen overshoot’ theory, which suggests that when oxygen- releasing microorganisms first began to flourish in the ocean, they hit a critical peak. Their supply of nutrients could no longer sustain them, leading to a decrease in atmospheric oxygen.
 The new data strengthen the idea that a nutrient famine is closely tied to atmospheric oxygen levels. “Some of these oxygen estimates likely require too many microorganisms living in the ocean in Earth’s past,” says geologist Peter Crockford from the Weizmann Institute of Science and Princeton University. “So we can now start to narrow in on what the composition of the atmosphere could have been through this biological angle.”
 If the team has it right, the deadly event that struck these microorganisms so long ago would have to be one of the most pronounced biotic changes across all of Earth’s history, even more so than the instances where as many as two-thirds of all plant and animal life disappeared.

(A)

Michelangelo: Although known today more for his painting and sculpture, Michelangelo was also a master architect. In fact, he was among the first to depart from the classical style and challenge traditional expectations.
 In 1523, Pope Clement VII commissioned Michelangelo to design a two-story library on top of an existing convent. Michelangelo employed radical principles in his design of the Laurentian Library in Florence, breaking rules of the classical style. For instance, he took practical elements, like brackets traditionally used as supportive structures, and used them merely for ornamentation.
 Michelangelo’s most famous contribution to architecture is probably the dome of St. Peter’s Basilica in Vatican City. It stands as one of the most recognizable landmarks in the world and inspired many imitators, such as the U.S. Capitol in Washington, D.C. However, the dome itself was not completed before Michelangelo died. Scholars still debate how the ultimate construction deviates from Michelangelo’s plans.

(B)

Imhotep Imhotep lived sometime between 2667 B.C.E. and 2648 B.C.E. Although he was born a commoner, Imhotep rose to become chief architec to Pharaoh Djoser of the Third Dynasty of Egypt and is known as the first architect, among other distinctions. Imhotep is credited with designing the Step Pyramid at Saqqara, a pharaoh’s tomb. According to Discovering Egypt, this, the world’s first pyramid, consisted of multiple mastabas (flat- roofed structures with sloping sides that had been the traditional pharaoh burial structures), one stacked atop the next becoming smaller each time. The result is a 62-meter-tall step structure surrounded by a massive complex that overlooked Memphis, the ancient capital. The tomb lies below the pyramid.
 To create the Step Pyramid, Imhotep invented new tools and equipment. While earlier mastabas had been made of clay brick, Imhotep used stone blocks, and the exterior was covered in limestone. Imhotep’s design influenced subsequent burial structures, culminating in the later pyramids like the Great Pyramid of Giza. Thanks to his ingenuity, Imhotep, who was also recognized as a healer, was later declared a god.

(C)

Sir Christopher Wren: Under normal circumstances, Sir Christopher Wren would probably be known as a great architect, but he might not have gone down in history as among the most famous architects who have ever lived. However, as it happened, Wren was in the right place at the right time and possessed the right talent.
 Wren was a professor of astronomy at Oxford who came to architecture through his interest in physics and engineering. In the 1660s, he was commissioned to design the Sheldonian Theatre at Oxford and visited Paris to study French and Italian baroque styles. In 1666, Wren completed a design for the dome of St. Paul’s Cathedral in London. However, one week after it was accepted, the Great Fire of London raged through the city, destroying most of it – including the cathedral.
 The Great Fire created an unexpected opportunity for Wren, who was soon at work on reconstruction. Although plans for a comprehensive reconstruction of the city soon proved too difficult, by 1669, Wren was appointed surveyor of royal works, which put him in charge of government building projects. Ultimately, he had his hand in designing 51 churches as well as St. Paul’s Cathedral.

(D)

Louis Henry Sullivan: Known for the principle of ‘form follows function’, Louis Henry Sullivan was anxious to break free from tradition and became influential in forging distinctly American architecture. Similar to Sir Christopher Wren, Sullivan benefited from a great fire. The Great Fire of 1871 in Chicago resulted in a construction boom and afforded architects like Sullivan with work for the decades to come. As a young man, he worked briefly in the offices of famed architects Frank Furness and then William Le Baron Jenney. He was only 24 years old when he became a partner in Dankmar Adler’s firm in 1881.
 As other architects like Jenney started implementing steel to allow for taller structures, the skyscraper was born. Sullivan was instrumental in creating a new functional design for these new, tall buildings rather than sticking with old-fashioned traditions. Because of this, some refer to Sullivan as the ‘Father of the Skyscraper’ (though others ascribe this title to Jenney). Sullivan’s designs also incorporated both geometric shapes and organic elements. His most famous work is the 10-story Wainwright Building in St. Louis, built in 1890, and the 16-story Guaranty Building in Buffalo, built in 1894.