Origin Story: A Big History of Everything | Chapter 13 of 31 - Part: 3 of 3

Author: David Christian | Submitted by: Maria Garcia | 5019 Views | Add a Review

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The composite Luca (or family of Lucas, because we’re really talking about billions of them) could adjust to changes in its environment. It had a genome, so it could reproduce. And it evolved. Luca may have lacked both its own membrane and its own metabolism. Its cell walls were probably made of porous volcanic rock, and its metabolism depended on geochemical flows of energy over which it had little control. The proteins Luca made suggest that it lived at the edge of alkaline oceanic vents, probably inside tiny pores in lavalike rocks, and it got its energy from nearby gradients of heat, acidity, and flows of protons and electrons. Luca’s chemical innards probably sloshed around in warm liquids from inside the Earth that were alkaline, which meant they had an excess of electrons. Just outside the volcanic pores Luca called home were cooler ocean waters that were more acidic, which meant they had an excess of protons. Like a charged battery, the tiny electrical gradient between Luca’s insides and the outer world provided the free energy needed to drive its metabolism, draw in nutrients from outside, and expel waste materials.

Here is how one of the pioneers of early life studies, Nick Lane, describes Luca:

She [Luca] was not a free-living cell but a rocky labyrinth of mineral cells, lined with catalytic walls composed of iron, sulphur and nickel, and energised by natural proton gradients. The first life was a porous rock that generated complex molecules and energy, right up to the formation of proteins and DNA itself.19

Though simple by comparison with modern organisms, Luca already contained a lot of neat biochemical gadgets, including many of the recipes for the metabolic and reproductive machinery of modern cells. It probably had a genome based on RNA so it could reproduce much more accurately and precisely than mere chemicals, and that suggests it may have been evolving fast. It was also using the energy flows it tapped to make ATP (adenosine triphosphate), the same molecule that transports energy inside modern cells.

From Luca to Prokaryotes

Luca and its relatives had already done a lot of the heavy lifting needed to evolve the first true living organisms. But Luca lacked a membrane that it could carry wherever it went, and a metabolism that was not tethered to energy flows near volcanic vents. Luca also seems to have lacked the more sophisticated reproductive mechanism that is present in most modern organisms and is based on RNA’s close relative, the double helix of DNA. At present, we know what had to evolve, but we do not understand the precise pathways by which these things evolved.

Explaining the evolution of personal protective membranes is not too difficult. Cell membranes are made from long chains of phospholipids, and it is not hard to persuade phospholipids to link up in layers that form semipermeable bubblelike structures under the right conditions. Perhaps, as Terrence Deacon has argued, autocatalytic reactions evolved and generated phospholipid layers, molecule by molecule. If so, it may not be too fanciful to imagine some version of Luca knitting itself a personal membrane.20

Explaining how cells evolved better ways of getting energy and reproducing is trickier, but the mechanisms involved are so fundamental and so elegant that it is worth trying to understand how they work.

Evolving new ways of tapping energy flows so that cells could move away from volcanic vents meant creating the cellular equivalent of an electricity grid that molecules could plug into as they went about their work. Enzymes played a crucial role here. These are specialist molecules that can act as catalysts, speeding up cellular reactions and reducing the activation energy needed to get them going. Today, enzymes play a fundamental role in all cells. Most enzymes are proteins, made from long chains of amino acids. The exact sequence of amino acids matters, because that determines how the protein will fold up into the precise shape it needs to do its particular job. Enzymes cruise through the molecular sludge, looking for target molecules that they fit on to, the way a wrench fits a particular nut or bolt. Then the enzyme uses tiny shots of energy to tap, bend, crack, or split the molecule, or bind it to other molecules. Most reactions in your body could not happen without enzymes or would require activation energies so high they would damage the cell.

Once the enzyme has knocked its target molecule into shape, it breaks away and goes hunting for other molecules that it can bend to its will. Enzymes can also be switched on or off by other molecules that bind to them and slightly alter their shape, and this is how, like billions of transistors in a computer, enzymes govern the fantastically complex reactions that go on inside cells.

Enzymes get the energy they need to do their work from the cellular equivalent of the electrical grid. This is a system that must have evolved very early in the history of life. Energy is carried to enzymes and other parts of the cell by molecules of ATP, or adenosine triphosphate, and ATP was probably hard at work already inside Luca. Enzymes and other molecules tap ATP’s energy by breaking off a small group of atoms, releasing the energy that binds that group to the molecule. The depleted molecule (now called ADP, for adenosine diphosphate) then heads off to special generator molecules that recharge it by replacing the lost atoms. The generator molecules are powered by a remarkable process called chemiosmosis, which was discovered only in the 1960s but seems to have been at work since the time of Luca. Inside each cell, food molecules are broken down to capture the energy they contain, and some of that energy is used to pump individual protons from inside the cell (where there is a low concentration of protons) to outside the cell (where there is a high concentration of protons). This is like charging a battery. It creates an electrical gradient between the outside and inside of the cell, with a voltage similar to what Luca may have used at alkaline vents. Special generator molecules (ATP synthase, for the technically minded) that are embedded in cell membranes use the electrical voltage created by protons returning from outside the membrane to drive nano-rotors. Like rotary assembly lines, the rotors charge up ADP molecules by replacing the group of molecules they have lost, then the charged-up ATP molecules go back into the cell and wait for other molecules to plug into them and get the energy they need to keep working.

This elegant cellular electrical grid is present in all cells today. It untethered cells from the energy flows around volcanic vents, allowing the earliest prokaryotes to roam Earth’s oceans, scrounging energy from food molecules and using them to create ATP molecules that could supply the energy needed to power the cell’s innards.

These delicate flows of energy maintained the complex inner structures of cells just as fusion maintains the structures of stars. Like fusion, they allowed the first living cells to pay the complexity taxes demanded by entropy, because in cells, as in stars, a lot of energy goes into keeping complex structures functioning. But also as in stars, a lot of energy is wasted because no reactions are 100 percent efficient, and of course, entropy loves wasted energy. In both cells and stars, concentrated flows of energy are needed to pay entropy’s taxes and overcome the universal tendency of all things to degrade.

In living organisms, energy has a new function that we don’t find in stars: it creates copies of the cell. These copies allow cells to push back against entropy by preserving their complex structures even after individual cells have died. Luca’s descendants evolved the elegant and efficient methods of reproduction that all living things still use today. Those methods are built on a key molecule, DNA, whose structure was first described in 1953 by Francis Crick and James Watson based on earlier research done by Rosalind Franklin. So much of evolution depends on understanding how DNA works that it is worth looking more carefully at this marvelous molecule.

DNA (deoxyribonucleic acid) is closely related to RNA (ribonucleic acid). Both are polymers, long chains of similar molecules. But while proteins are made from strings of amino acids, and membranes are made from phospholipids, DNA and RNA are made from long strings of nucleotides. These are sugar molecules to which are attached small groups of molecules known as bases. The bases come in four types: adenine (A), cytosine (C), guanine (G), and thymine (T). (In RNA, thymine is replaced by uracil, U.) And here’s the magic. As Crick and Watson showed, these four bases can be used like the letters of an alphabet to carry huge amounts of information. As DNA or RNA molecules link up to form huge chains, the bases stick out to the side, forming a long string of As, Cs, Gs, and Ts (or Us in RNA). Every group of three letters codes for a particular amino acid or contains an instruction, such as Stop reading now. Thus, the sequence TTA says, Add on a molecule of the amino acid leucine, while TAG is a sort of punctuation mark that says, Okay, you can stop copying now.

The information on DNA and RNA molecules can be read and copied because the bases like to link up with each other using hydrogen bonds, which can be made and broken quite easily. But they bond only in very specific ways. A always joins with T (or U in RNA), and C with G. Special enzymes expose stretches of DNA that correspond to a particular gene or code for a particular protein, and each base attracts its opposite to create a new short RNA chain of nucleotides that is complementary to the original chain. The newly created segment is then whisked off to a large molecule known as a ribosome, which is a sort of protein factory. The ribosome reads the sequence of letters in triplets and extrudes the corresponding amino acids, one by one, in just the right order to make a particular protein, which then goes off into the cell to do its work. In this way, ribosomes can manufacture all of the thousands of proteins a cell needs.

The final piece of magic is that DNA and RNA molecules can use these copying mechanisms to make copies of themselves and all the information they contain. The bases that stick out sideways from their sugar-phosphate chains reach into the cellular sludge and grab onto their complements. Thus, Cs always grab onto Gs, and As always grab Ts (or Us, in RNA). The newly attached bases attract new sugar molecules that link together, and in this way they form a new chain that is the exact complement of the first. In DNA, these two complementary chains normally stick together, which is why DNA usually exists in the form of a double chain or helix, like a pair of winding staircases. It can be wound up so tightly that it packs neatly inside each cell, and it is unwound only to be read or to make copies of itself. However, RNA normally exists as a single chain, so, like a protein, it can also fold up into particular shapes and function like an enzyme.

This small difference between RNA and DNA is hugely important because it means that, while DNA normally functions just as a store of genetic information, RNA can both store information and do chemical work. It is both hardware and software, and that is why most researchers believe that there was a time, perhaps when Luca was still around, when most genetic information was carried by RNA. Luca probably lived in such an RNA world. But RNA is a less secure information carrier than DNA because its information is constantly buffeted in the violent inner world of the cell, whereas the double strands of DNA shield their precious information from the whirlwind outside. In an RNA world, genetic information could easily get lost or distorted. Evolution really got going only after the development of a DNA world by Luca’s descendants, the true prokaryotes, which dominate the world of microorganisms today.

With membranes of their own, an independent metabolism, and more precise and stable genetic machinery, the first prokaryotes could leave the volcanic vents in which they had been born and cruise the oceans of the early Earth. They were probably already doing this 3.8 billion years ago.

Each prokaryote is an entire kingdom of staggering complexity. Billions of molecules swim through a thick chemical slurry, being nudged and pulled by other molecules thousands of times each second, rather like a tourist in a crowded market full of traders, touts, and pickpockets. If you were injected into one of these molecules, you would find this a terrifying world. Enzymes will try to glom on to you and change you, perhaps hook you up with other molecules to form a new team that can cruise the markets looking for new opportunities. Imagine millions of these interactions going on inside every cell every second and you have some idea of the frenetic activity that powers even the simplest of cells in the early biosphere.

This is a new world and a new kind of complexity. Like stars and planets formed during periods of chaotic change, cells eventually settled into a sort of stability as they began to manage and push back against tiny fluctuations in their environments. Cells would achieve a temporary balance; so, too, would entire species and lineages and groups of species. But this was never a static balance. It was always dynamic, always maintained by a constant negotiation between living organisms and changing environments, and always in danger of a sudden breakdown.

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Alice
Great book, nicely written and thank you BooksVooks for uploading

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