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Building blocks that fall from the sky How did life on Earth begin? Scientists from the “Heidelberg Initiative for the Origin of Life” have set about answering this truly existential question. Indeed, they are going one step further and examining the conditions under which life can emerge. The initiative was founded by Thomas Henning, Director at the Max Planck Institute for Astronomy in Heidelberg, and brings together researchers from chemistry, physics and the geological and biological sciences. 18 MaxPlanckResearch 3 | 18
FOCUS_The Origin of Life
TEXT THOMAS BUEHRKE
T
he great questions of our exis- However, recent developments are The initiative was triggered by the dis-
tence are the ones that fasci- forcing researchers to break down this covery of an ever greater number of
nate us the most: how did the specialization and combine different rocky planets orbiting around stars oth-
universe evolve, and how did disciplines. “That’s what we’re trying er than the Sun. “We now know that
Earth form and life begin? to do with the Heidelberg Initiative terrestrial planets of this kind are more
Does life exist anywhere else, or are we for the Origins of Life, which was commonplace than the Jupiter-like gas
alone in the vastness of space? By ap- founded three years ago,” says Thom- giants we identified initially,” says Hen-
proaching these puzzles from various as Henning. HIFOL, as the initiative’s ning. Accordingly, our Milky Way alone
angles, scientists can answer different as- name is abbreviated, not only incor- is home to billions of rocky planets,
pects of this question. For a long time, porates researchers from different dis- some of which presumably offer envi-
there was a clear division of tasks: astro- ciplines, but also cooperates closely ronmental conditions that favor the
physicists and geophysicists were respon- with international institutions such emergence of life as we know it. It is
sible for studying the universe and Earth, as the McMaster University in Ham- precisely this realization that broadens
biologists and chemists for studying life. ilton, Canada. the scope of the Heidelberg Initiative: it
Precious cargo: meteorites may have brought key chemicals – such as the nucleobases
adenine, guanine and uracil – to Earth, thereby providing the ingredients for the formation
of RNA molecules.
Photo: DLR
3 | 18 MaxPlanckResearch 19
Cosmic nurseries: stars and planets form in clouds of gas and dust. The image above shows three such areas: the Omega and Eagle Nebulae and
the Sharpless 2-54 complex (from left). This process of formation not only gave rise to our solar system some 4.6 billion years ago, but also continues
to take place at many locations in space. It is also responsible for producing the red dwarf star Trappist-1, which is located 40 light years from Earth,
and the seven relatively Earth-like rocky planets that have been identified in its system so far (right).
must ask not only how life could have challenged the study of the astrono- istry. This states that the first ever ter-
emerged on Earth but also, more gen- mers following the thought: what do restrial life forms were based on ribonu-
erally, what conditions are needed for astronomers understand about biomol- cleic acids (RNA). Structurally, RNA
something like this to happen – includ- ecules – even if they have a deep knowl- resembles DNA, the information carri-
ing on extrasolar planets. edge of astrochemistry? The National er of life today: both are made up of
Academy of Science took a different four organic bases, including adenine,
PRAISE AND CRITICISM FOR view, however, and awarded the publi- guanine and cytosine; however, RNA
AN EXCELLENT STUDY cation the 2017 Cozzarelli Prize for its contains a base known as uracil instead
“outstanding scientific excellence and of thymine. In addition, RNA is typical-
Late last year, Thomas Henning and his originality”. In fact, astronomers can ly single-stranded – unlike the dou-
colleague Dmitry Semenov, as well as indeed contribute to questions of what ble-stranded structure of DNA.
Ben Pearce and Ralph Pudritz from Mc- conditions potentially existed when Likewise, molecules of RNA can also
Master University, caused quite a stir the first molecules of life or their pre- communicate genetic information and
with a publication in which they pro- cursors were formed – and of how this perform catalytic functions. In most or-
posed a scenario for the emergence of came about. ganisms, however, RNA serves as an in-
life on Earth. “As well as recognition As a starting point for the study, formation carrier in a subordinate role
and praise, it also brought us some crit- Henning and his colleagues applied the to DNA, only acting as a storage medi-
icism,” says Henning. widely used “RNA world” hypothesis um in viruses. Did the more complex
Photo: ESO
The criticism came from the more proposed some 30 years ago by Walter DNA therefore evolve from simpler
traditional origins-of-life scientists, who Gilbert, a Nobel Prize winner in Chem- RNA molecules or related biopolymers?
20 MaxPlanckResearch 3 | 18
FOCUS_The Origin of Life
In 2009, a chemical experiment by Brit- Indeed, the nucleobases adenine, gua- There are almost no remnants of the
ish researchers was considered a major nine and uracil, as well as amino acids, very early phase in which Earth cooled
breakthrough, for it showed that RNA have been detected inside meteorites. from a glowing ball into a rocky plan-
building blocks can form if certain mol- These bases are formed from simple et. Only the presence of tiny zircon
ecules are present and react with one molecules of hydrogen cyanide, carbon crystals, which could be as much as 4.4
another under very specific conditions. monoxide and ammonia in the pres- billion years old, suggests that Earth
But where were the most favorable con- ence of water. formed a solid crust at quite an early
ditions present in nature? stage. At that time, Earth was exposed
For a long time, it has been suspect- ZIRCON CRYSTALS to a much heavier bombardment of
ed that life emerged at hydrothermal INDICATE A SOLID CRUST meteorites than it is today, as is demon-
vents – so-called black and white smok- strated by crater statistics for the Moon,
ers – on the deep seafloor. However, it Meteorites have also been found to con- which received the same onslaught.
is unclear whether the nitrogen need- tain the mineral Schreibersite, which re- The cosmic projectiles presumably
ed for synthesis exists at a sufficient leases phosphorus capable of forming brought both water and organic mole-
concentration in that environment. phosphorylated molecules in water – cules to our planet.
Moreover, the substances are diluted by these too are required for RNA synthe- Just as little is known about the dis-
a steady flow of water, which prevents sis. Astronomical observations have tribution of land and water in primeval
complex chemical reactions from tak- shown that all of the necessary precur- times as it is about the temperature, for
ing place. sor materials to form RNA are present in example, which is a crucial factor in
“This is where we come into play,” the dust discs in which planets form, so chemical reactions. For this reason, the
Photo: M. Kornmesser/ESO
says Thomas Henning. “We asked our- they must also have been present in the astronomers calculated models in which
selves what other geochemical condi- solar nebula that gave birth to our solar they varied key parameters of the devel-
tions might have been present to allow system 4.6 billion years ago. But how oping crust over a wide range of values.
this RNA synthesis to take place.” The and when did the organic building Earth at the time undoubtedly had wa-
idea is that the most important build- blocks arrive on Earth? And what did ter reservoirs with a wide range of siz-
ing blocks came to Earth from space. Earth look like at the time? es, just as it does now. >
3 | 18 MaxPlanckResearch 21
Weak Heavy
rainfall rainfall
UV light
Meteorites 0.4 Wm -2 Meteorites
~ 2-20 cm ~ 2-20 cm
Evaporation
Photodissociation 1-10 m
Hydrolysis
1-10 m
The cradle of life: Dmitry Semenov (left)
Percolation and Thomas Henning from the Max
Planck Institute for Astronomy have
outlined a scenario in which various
Dry Wet
reactions once occurred in small, warm
pools. These reactions led to the
formation of the first self-replicating
S e a s o n al c y c l e RNA molecules. The diagram on the left
shows the numerous influences that
affected chemical compounds in these
small bodies of water.
Large lakes and seas were presumably and then fill up with water again may strated in impressive fashion, meteorites
unsuitable breeding sites for RNA, be- have favored the formation of longer of this order of magnitude do not make
cause the precursors must be present in RNA chains,” says Henning. it to the ground unscathed – rather, they
concentrated form in order to react break up into many small fragments and
with one another. However, the model THE IDEAL RADIUS IS fall to Earth over a large area. This means
showed that small pools with a diame- BETWEEN 40 AND 80 METERS it is possible for tiny pieces, measuring
ter and depth of just a few meters were just a few centimeters across, to have
ideal: they were large enough not to dry In the model simulations, the research- landed in the pools. Depending on their
out too quickly, yet small enough to al- ers also varied the impact rate and size size, they would then have released the
low high nucleobase concentrations to distribution of the meteorites. If these nucleobases over a period of several days
accumulate rapidly. are too small, they burn up completely to months. After that, the nucleotides –
At the same time, the biomolecules in the atmosphere; if they are too large, and the RNA molecules formed from
were at the mercy of destructive influ- they hit the ground with too much ki- them – would have to be synthesized
ences: in the water, they were endan- netic energy. “A radius of between 40 within a few years.
gered by electrolysis and, in the open, and 80 meters is the optimum size to The simulations show that meteor-
by the intensive UV radiation of the allow the meteorites to deliver their ites could have transported a sufficient
sun. Some 95 per cent of UV radiation molecular payload to the ground,” ex- quantity of nucleobases, to small pools
Graphic: McMaster University
is absorbed by a layer of water just one plains Dmitry Semenov, an expert in on Earth and thereby triggered the for-
meter thick. The optimum scenario chemical networks within protoplane- mation of RNA molecules in at least
seems to be one where the pools expe- tary discs and co-author of the study. one such pool. The RNA world could
rienced seasonal variations in fill level This range is two to four times the have emerged within 200 to 300 mil-
due to rainfall and drying out through size of the meteorite that exploded over lion years from the point at which
evaporation and percolation: “The cy- the Russian city of Chelyabinsk in Feb- Earth’s surface became habitable – that
cles in which shallow pools first dry out ruary 2013. As that incident demon- is, over four billion years ago.
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FOCUS_The Origin of Life
“Based on what we know about planet supplied by flashes of lightning. After has become a Max Planck Fellow: the
formation and the chemistry of the so- some time, they were able to detect or- Max Planck Society supports him with
lar system, we’ve proposed a consistent ganic molecules, including amino ac- research funding and finances part of
scenario for the origins of life on Earth,” ids, using a chromatograph. his 16-person group.
says Semenov. “Now, the experimental- Today, however, researchers assume “In our chemical experiments, we
ists need to work out how life could ac- that Earth’s primordial atmosphere had reproduce the conditions stipulated to
tually have emerged under these very a different composition to that as- us by the astrophysicists,” says Trapp.
specific early conditions.” In fact, the sumed by Miller and Urey, containing The researchers therefore take real me-
nucleobases are just the first step. Oth- less methane and instead higher levels teorite material and place it in a reac-
er necessary processes include, for ex- of hydrogen, carbon dioxide, nitrogen tor, where its reactions then produce
ample, the formation of complex RNA- and water. In these conditions, it was numerous organic molecules that can
like molecules, cell membranes and probably more difficult to synthesize be analyzed using fast chromatograph-
ultimately the DNA-protein world of the necessary building blocks for RNA. ic techniques. The results are surpris-
today’s organisms. ing: the tiny meteorite particles on the
It is impossible to talk about chem- GENUINE METEORITE MATERIAL nanometer scale act as catalysts for
Photo: Axel M. Quetz/MPI for Astronomy
ical experiments relating to the origins REACTING IN A REACTOR these reactions.
of life without mentioning the famous Interestingly, substances formed in
Miller-Urey experiment in the 1950s. The job of investigating how this could the process then act as catalysts them-
Stanley Miller and Harold Clayton Urey have been possible falls to Oliver Trapp, selves, speeding up the production of
placed simple chemical substances who carried out research at the Univer- the same or even other substances. The
within a hypothetical early Earth atmo- sity of Heidelberg before accepting a reaction enters a state of dynamic im-
sphere inside a reaction vessel. They professorship at LMU Munich. In order balance: only the substances that form
then exposed them to electrical dis- to maintain his productive collabora- catalytically the fastest are able to accu-
charges in order to mimic the energy tion with the team in Heidelberg, he mulate in large enough quantities. “A
3 | 18 MaxPlanckResearch 23
FOCUS_The Origin of Life
sort of chemical evolution takes place,” pered by a problem that has been hith-
says Oliver Trapp. “The aim is to see erto unknown – one that affects planets
whether this chemical evolution leads orbiting cool, low-luminosity stars
to an RNA world.” In these experi- known as “M dwarfs”. The latest exam-
ments, it has also become apparent that ples of such planets are Proxima Centau-
a kind of motor is needed to drive the ri b and Trappist-1d. In these systems,
reactions: the natural light/dark cycle the habitable zone lies much closer to
of day and night. In other experiments, the star than is the case with our hotter
Trapp’s group is studying fatty acids Sun. A potentially inhabited planet will
and the question of how cell mem- therefore presumably exhibit bound ro-
branes could have formed. tation, in which the same hemisphere
The analysis of chemical reactions always faces the star, resulting in perpet-
in different conditions requires the use ual day on one half of the planet and
of high-throughput screening tech- perpetual night on the other.
niques, which allow Trapp’s laboratory Computer simulations by a research
to run and analyze chemical processes group led by Ludmila Carone from the
in 64 minireactors, each with a capaci- Max Planck Institute for Astronomy in
ty of just 1.5 milliliters. If certain con- Heidelberg show that a specific air cur-
ditions prove especially promising at rent forms in the atmosphere of such
Evolution in the lab: working with Harold
this stage, they are then studied in planets. This current causes the ozone
Clayton Urey in the 1950s, Stanley Miller greater detail in two-liter reactors. “We to accumulate in the equatorial region,
(pictured) succeeded in producing organic jokingly refer to this as our Urey-Miller while it is practically absent in all oth-
molecules in a reaction vessel – including 2.0,” says Trapp. er regions. “If we can’t detect ozone on
amino acids. This famous experiment is
a distant planet, that doesn’t necessar-
today inspiring researchers to carry out
further tests. SEARCHING FOR BIOMARKERS ily mean there’s no oxygen there at all,”
IN THE ATMOSPHERE Carone explains. “We might simply
have looked in the wrong place – and
The reactions are very complex, and the ozone is hiding elsewhere.”
the researchers are only just beginning Nevertheless, astronomers are
to address many of the questions. How- searching for a potential second Earth
ever, Oliver Trapp believes that the in M dwarf systems too, especially as
emergence of life is inevitable if the these are much more common than
conditions are right. “I’m even con- stars that resemble our Sun. The Hei-
vinced that the chemical structure of delberg Max Planck researchers have
potential extraterrestrial life will close- also been on a very special planetary
ly resemble that of life on Earth.” hunt for a good two years now. Work-
Photo: Special Collections & Archives, UC San Diego Library
This also raises the question of ing with colleagues from other Ger-
whether we will be able to detect the man and Spanish institutes, they have
activity of life on another planet. The built an instrument that is studying
corresponding biomarkers are general- around 300 M dwarfs using the largest
ly considered to be molecular oxygen, telescope at Calar Alto Observatory in
ozone and methane. However, it is im- southern Spain and looking out for
portant to bear in mind that the oxy- signs of rocky planets.
gen concentration on Earth did not However, the James Webb Space
reach its current value until approxi- Telescope is another source of great
mately 300 million years ago. That is a hope for astronomers. Launching in
relatively short time frame in terms of two years’ time, at the earliest, this suc-
biological evolution. cessor to Hubble will travel millions of
In addition, detecting these sub- kilometers from Earth on its mission to
stances on an exoplanet could be ham- explore the universe. One of the prima-
24 MaxPlanckResearch 3 | 18
A gigantic compound eye: the gold-sputtered main mirror of the James Webb Space Telescope has a diameter of six and a half meters and is made up of
18 segments. From May 2020, at the earliest, the instrument will scour the heavens and train its sights on distant exoplanets.
ry objectives is to study exoplanets. The SUMMARY
Max Planck Institute for Astronomy de- l Today’s life is based on the genetic information carrier DNA. As a forerunner
veloped and built key components for to this, a world based on the simpler biomolecule RNA could have existed on
one of the four measurement instru- the early Earth.
ments, which goes by the name of l T he building blocks for RNA might have arrived on Earth inside meteorites.
MIRI, and so the Heidelberg astrono- Computer simulations suggest that the subsequent synthesis of RNA began
mers will have the chance to make in pools measuring just a few meters in diameter.
some of the first observations using the l Using high-throughput screening, it is possible to test a very large number
of chemical reactions in a short time and thus to narrow down the optimum
super telescope.
conditions for the emergence of life.
Like his colleague Oliver Trapp,
Thomas Henning is also confident of
the existence of life on faraway plan-
ets: “Simply by virtue of the enormous GLOSSARY
number of rocky planets in our Milky Electrolysis: Splitting of a chemical compound under the action of electrical current,
Way – perhaps as many as a billion – resulting in the conversion of electrical energy into chemical energy. Electrolysis is the
and the knowledge that life emerged reverse reaction to that taking place in a battery or fuel cell.
very quickly on Earth, I think it’s very Exoplanet: A planet outside the gravitational field of our Sun but within the gravitation-
likely that life exists on other planets.” al field of another star. Around 3,800 exoplanets are currently known to astronomers.
Are we therefore heading for a second
Photo: NASA/Desiree Stover
Nucleobases: Constituents of nucleic acids such as DNA or RNA. DNA contains the
Copernican Revolution? “No,” replies nucleobases adenine, guanine, cytosine and thymine; in molecules of RNA, thymine is
substituted for uracil. They are called bases because they dissolve in water to form
Henning, “we’re already in the midst
weakly basic solutions. Adenine and thymine, as well as guanine and cytosine, each form
of it.” base pairs, which then combine with the sugar molecule deoxyribose and a phosphate
group to produce the basic backbone of the DNA double helix.
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3 | 18 MaxPlanckResearch 25
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