OUR VIRALWORLD

By Jason Treat, Mesa Schumacher,

and Eve Conant

Illustrations By Markos Kay

PublisheD January 14, 2021

Cells are considered the foundation of life, but viruses—with all their genetic diversity—may share in that role. Our planet’s earliest viruses and cells likely evolved in an intertwined and often symbiotic relationship of predator and prey. Evidence even suggests that viruses may have started out as cells but lost their autonomy as they evolved to thrive as parasites on other cells. This dependent relationship began a long history of coevolution. Viruses living in cells cause their hosts to adapt, and those changes then cause viruses to adapt in a never ending cycle of one-upmanship.

A more inclusive

tree of life?

Billions of years ago life on Earth diverged into three branches: Archaea, Bacteria, and Eukarya. But recent research suggests viruses should be considered a fourth branch. This diagram, based on comparing the shapes of proteins in viruses and cellular organisms, shows that viruses share many primitive characteristics with early cellular ancestors—and likely evolved alongside them.

In this diagram, the distance between viruses and organisms indicates how closely related they are to each other.

Archaea

Single-cell organisms that share some characteristics with Eukarya and Bacteria and thrive in extreme environments

Eukarya

Single- and multicellular organisms whose cells have an organized nucleus

Cellular

organism

Bacteria

Single-cell organisms lacking the organized nucleus of Eukarya

Viruses

From Small and Simple to Large and Complex

Viruses are shown here at 10,000 to 20,000 times actual size, depending on screen size.

Adeno-associated virus

20 nanometers (nm)

in diameter

Magnified x10

Capsid

DNA

A Useful Tool

Scientists are now able to insert DNA into the genome of many rudimentary viruses, then use them to deliver this material to specific cells. This promising research may lead to safer methods of gene therapy.

Marine viruses

These tiny viruses infect ammonia-­oxidizing Archaea, oceanic microorganisms that play a major role in carbon and nitrogen cycling. Regular infection of Archaea by these viruses may help regulate these cycles, impacting entire ecosystems.

Nitrosopumilus

spindle-shaped

virus (NSV)

65 nm

About 55 million Zika viruses could fit on the period at the end of this sentence.

Zika

50 nm

Mosquito-borne viruses

Medium-size viruses like West Nile and dengue are transmitted to humans via saliva in a mosquito’s bite and travel throughout the body in the bloodstream.

West Nile

50 nm

Dengue

50 nm

Arenavirus

80-100 nm

Drivers of Evolution

Rodent-borne arenaviruses typically

pass through a cell’s membrane by using receptors that import iron into cells. In response to the viruses, these receptors have been continually modified, an example of how viruses shape the evolution of life.

Influenza

80-100 nm

Measles

80-100 nm

Viral Instructors

Vaccination is like a training exercise for the immune system. Exposure to a weakened virus, dead virus, or a component of a virus teaches the body to recognize and attack that specific invader. If the virus is encountered, the immune system will be able to respond faster. Measles and influenza are viruses successfully controlled with vaccines.

Human

immunodeficiency

virus (HIV)

80-100 nm

Viral Editors

Viruses enter a cell and hijack its machinery to replicate, but retroviruses such as HIV do it by inserting their genes into the cell’s DNA. If they do it in germ cells, the DNA can end up as part of the host’s genome. There are thousands of fragments of ancient retroviruses in the human genome. Scientists found that a crucial membrane in the mammalian placenta—which makes internal pregnancy possible—

evolved with the help of ancient retroviral genes.

SARS-CoV-1

80-120 nm

Coronaviruses

There are seven coronaviruses known to infect humans; one has led to the current pandemic. Named for their spiky proteins—“corona” is Latin for crown—that help them attack cells, they spread through respiratory droplets and aerosols.

Spike

protein

RNA

Envelope

SARS-CoV-2

(causes COVID-19)

120 nm

MERS-CoV

120-135 nm

Rabies

75 x 180 nm

Variola

(causes smallpox)

325 x 260 nm

Deadly adaptors

Large viruses that infect humans, such as variola (which causes smallpox) and Ebola virus, tend to have a high death rate. This is probably because many larger viruses carry viral proteins, in addition to their genes, that overwhelm and shut down the host’s defenses.

Ebola virus

970 x 80 nm

Bacteriophages

As viruses get larger, they also get more sophisticated. Bacteriophages are viruses that target bacteria; this one (at left) infects E. coli. Bacteriophages have complex heads that carry DNA and intricate tail structures that identify and bind to host cells, then inject viral genetic material through specialized tubes.

Bacteriophage T4

90 x 200 nm

Mimivirus

750 nm

Big Clues to

Early Origins

The reduction hypothesis supposes that viruses downsized in favor of using their host’s machinery to reproduce. New support for the idea arrived in the form of the giant viruses of the Mimiviridae family. They set up “virus factories” in host cells that may resemble some of the earliest virus-cell interactions.

Sources: Gustavo Caetano-Anollés, University of Illinois at Urbana-Champaign; Mya Breitbart, University of South Florida; Edward Chuong, University of Colorado Boulder

By Jason Treat, Mesa Schumacher, and Eve Conant

Illustrations By Markos Kay

PublisheD January 14, 2021

Cells are considered the foundation of life, but viruses—with all their genetic diversity—may share in that role. Our planet’s earliest viruses and cells likely evolved in an intertwined and often symbiotic relationship of predator and prey. Evidence even suggests that viruses may have started out as cells but lost their autonomy as they evolved to thrive as parasites on other cells. This dependent relationship began a long history of coevolution. Viruses living in cells cause their hosts to adapt, and those changes then cause viruses to adapt in a never ending cycle of one-upmanship.

A more inclusive tree of life?

Billions of years ago life on Earth diverged into three branches: Archaea, Bacteria, and Eukarya. But recent research suggests viruses should be considered a fourth branch. This diagram, based on comparing the shapes of proteins in viruses and cellular organisms, shows that viruses share many primitive characteristics with early cellular ancestors—and likely evolved alongside them.

Archaea

Single-cell organisms that share some characteristics with Eukarya and Bacteria and thrive in extreme environments

Eukarya

Single- and multicellular organisms whose cells have an organized nucleus

Bacteria

Single-cell organisms lacking the organized nucleus of Eukarya

Cellular

organism

In this diagram, the distance between viruses and organisms indicates how closely related they are to each other.

Viruses

From Small and Simple

to Large and Complex

Viruses are shown here at 10,000 to 20,000 times actual size, depending on screen size.

Adeno-associated virus

20 nanometers (nm)

in diameter

A Useful Tool

Scientists are now able to insert DNA into the genome of many rudimentary viruses, then use them to deliver this material to specific cells. This promising research may lead to safer methods of gene therapy.

Magnified x10

Capsid

DNA

Marine viruses

These tiny viruses infect ammonia-­

oxidizing Archaea, oceanic microorganisms that play a major role in carbon and nitrogen cycling. Regular infection of Archaea by these viruses may help regulate these cycles, impacting entire ecosystems.

Nitrosopumilus

spindle-shaped virus (NSV)

65 nm

About 55 million Zika viruses could fit on the period at the end of this sentence.

Zika

50 nm

Mosquito-borne viruses

Medium-size viruses like West Nile and dengue are transmitted to humans via saliva in a mosquito’s bite and travel throughout the body in the bloodstream.

West Nile

50 nm

Dengue

50 nm

Drivers of Evolution

Rodent-borne arenaviruses typically pass through a cell’s membrane by using receptors that import iron into cells. In response to the viruses, these receptors have been continually modified, an example of how viruses shape the evolution of life.

Arenavirus

80-100 nm

Viral Instructors

Vaccination is like a training exercise for the immune system. Exposure to a weakened virus, dead virus, or a component of a virus teaches the body to recognize and attack that specific invader. If the virus is encountered, the immune system will be able to respond faster. Measles and influenza are viruses successfully controlled with vaccines.

Influenza

80-100 nm

Measles

80-100 nm

Viral Editors

Viruses enter a cell and hijack its machinery to replicate, but retroviruses such as HIV do it by inserting their genes into the cell’s DNA. If they do it in germ cells, the DNA can end up as part of the host’s genome. There are thousands of fragments of ancient retroviruses in the human genome. Scientists found that a crucial membrane in the mammalian placenta—which makes internal pregnancy possible—evolved with the help of ancient retroviral genes.

Human

immunodeficiency

virus (HIV)

80-100 nm

SARS-CoV-1

80-120 nm

RNA

Envelope

Spike protein

Coronaviruses

There are seven coronaviruses known to infect humans; one has led to the current pandemic. Named for their spiky proteins—“corona” is Latin for crown—that help them attack cells, they spread through respiratory droplets and aerosols.

SARS-CoV-2

(causes COVID-19)

120 nm

MERS-CoV

120-135 nm

Ebola virus

970 x 80 nm

Rabies

75 x 180 nm

Deadly adaptors

Large viruses that infect humans, such as variola (which causes smallpox) and Ebola virus, tend to have a high death rate. This is probably because many larger viruses carry viral proteins, in addition to their genes, that overwhelm and shut down the host’s defenses.

Variola

(causes smallpox)

325 x 260 nm

Bacteriophage T4

90 x 200 nm

Bacteriophages

As viruses get larger, they also get more sophisticated. Bacteriophages are viruses that target bacteria; this one (at right) infects E. coli. Bacteriophages have complex heads that carry DNA and intricate tail structures that identify and bind to host cells, then inject viral genetic material through specialized tubes.

Big Clues to Early Origins

The reduction hypothesis supposes that viruses downsized in favor of using their host’s machinery to reproduce. New support for the idea arrived in the form of the giant viruses of the Mimiviridae family. They set up “virus factories” in host cells that may resemble some of the earliest virus-cell interactions.

Mimivirus

750 nm

Sources: Gustavo Caetano-Anollés, University of Illinois at Urbana-Champaign; Mya Breitbart, University of South Florida; Edward Chuong, University of Colorado Boulder

By Jason Treat, Mesa Schumacher, and Eve Conant

Illustrations By Markos Kay

PublisheD January 14, 2021

Cells are considered the foundation of life, but viruses—with all their genetic diversity—may share in that role. Our planet’s earliest viruses and cells likely evolved in an intertwined and often symbiotic relationship of predator and prey. Evidence even suggests that viruses may have started out as cells but lost their autonomy as they evolved to thrive as parasites on other cells. This dependent relationship began a long history of coevolution. Viruses living in cells cause their hosts to adapt, and those changes then cause viruses to adapt in a never ending cycle of one-upmanship.

A more inclusive tree of life?

Billions of years ago life on Earth diverged into three branches: Archaea, Bacteria, and Eukarya. But recent research suggests viruses should be considered a fourth branch. This diagram, based on comparing the shapes of proteins in viruses and cellular organisms, shows that viruses share many primitive characteristics with early cellular ancestors—and likely evolved alongside them.

Archaea

Single-cell organisms that share some characteristics with Eukarya and Bacteria and thrive in extreme environments

Eukarya

Single- and multicellular organisms whose cells have an organized nucleus

Bacteria

Single-cell organisms lacking the organized nucleus of Eukarya

Cellular

organism

In this diagram, the distance between viruses and organisms indicates how closely related they are to each other.

Viruses

From Small and Simple

to Large and Complex

Viruses are shown here at 10,000 to 20,000 times actual size, depending on screen size.

Adeno-associated virus

20 nanometers (nm)

in diameter

A Useful Tool

Scientists are now able to insert DNA into the genome of many rudimentary viruses, then use them to deliver this material to specific cells. This promising research may lead to safer methods of gene therapy.

Magnified x10

Capsid

DNA

Marine viruses

These tiny viruses infect ammonia-­

oxidizing Archaea, oceanic microorganisms that play a major role in carbon and nitrogen cycling. Regular infection of Archaea by these viruses may help regulate these cycles, impacting entire ecosystems.

Nitrosopumilus

spindle-shaped virus (NSV)

65 nm

About 55 million Zika viruses could fit on the period at the end of this sentence.

Zika

50 nm

Mosquito-borne viruses

Medium-size viruses like West Nile and dengue are transmitted to humans via saliva in a mosquito’s bite and travel throughout the body in the bloodstream.

West Nile

50 nm

Dengue

50 nm

Drivers of Evolution

Rodent-borne arenaviruses typically pass

through a cell’s membrane by using receptors

that import iron into cells. In response to the

viruses, these receptors have been

continually modified, an example of how

viruses shape the evolution of life.

Arenavirus

80-100 nm

Viral Instructors

Vaccination is like a training exercise for the immune system. Exposure to a weakened virus, dead virus, or a component of a virus teaches the body to recognize and attack that specific invader. If the virus is encountered, the immune system will be able to respond faster. Measles and influenza are viruses successfully controlled with vaccines.

Influenza

80-100 nm

Measles

80-100 nm

Viral Editors

Viruses enter a cell and hijack its machinery to replicate, but retroviruses such as HIV do it by inserting their genes into the cell’s DNA. If they do it in germ cells, the DNA can end up as part of the host’s genome. There are thousands of fragments of ancient retroviruses in the human genome. Scientists found that a crucial membrane in the mammalian placenta—which makes internal pregnancy possible—evolved with the help of ancient retroviral genes.

Human

immunodeficiency

virus (HIV)

80-100 nm

SARS-CoV-1

80-120 nm

RNA

Envelope

Spike protein

Coronaviruses

There are seven coronaviruses known to infect humans; one has led to the current pandemic. Named for their spiky proteins—“corona” is Latin for crown—that help them attack cells, they spread through respiratory droplets and aerosols.

SARS-CoV-2

(causes COVID-19)

120 nm

MERS-CoV

120-135 nm

Rabies

75 x 180 nm

Deadly adaptors

Large viruses that infect humans, such as variola (which causes smallpox) and Ebola virus, tend to have a high death rate. This is probably because many larger viruses carry viral proteins, in addition to their genes, that overwhelm and shut down the host’s defenses.

Ebola virus

970 x 80 nm

Variola

(causes smallpox)

325 x 260 nm

Bacteriophage T4

90 x 200 nm

Bacteriophages

As viruses get larger, they also get more sophisticated. Bacteriophages are viruses that target bacteria; this one (at right) infects E. coli. Bacteriophages have complex heads that carry DNA and intricate tail structures that identify and bind to host cells, then inject viral genetic material through specialized tubes.

Big Clues to Early Origins

The reduction hypothesis supposes that viruses downsized in favor of using their host’s machinery to reproduce. New support for the idea arrived in the form of the giant viruses of the Mimiviridae family. They set up “virus factories” in host cells that may resemble some of the earliest virus-cell interactions.

Mimivirus

750 nm

Sources: Gustavo Caetano-Anollés, University of Illinois at Urbana-Champaign; Mya Breitbart, University of South Florida; Edward Chuong, University of Colorado Boulder