Ancient life: discussing success and extinction

B2
90 min
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1

Think about these questions before watching. Share your ideas with a partner.

  1. When you think of 'successful' species, which ones come to mind and what criteria are you using to judge their success?
  2. Considering the vast timescales of evolution, what do you think the legacy of humanity will be on the planet's fossil record millions of years from now?
  3. If you could witness one period in Earth's prehistoric past, which would you choose and why? What specific mysteries about the life of that era would you hope to solve?
2

Watch the video carefully. Pay attention to the main ideas and key details.

Video script76 segments · click a timestamp to jump

A procession of segmented creatures moves across the sea floor.

Up ahead, hundreds have begun shedding their exoskeletons

and piling on top of one another in what appears to be... a massive orgy.

But this is not some alien world.

It’s Earth about 500 million years ago,

when these creatures, called trilobites, prospered.

Prevailing for around 270 million years

and encompassing more than 20,000 distinct species,

trilobites are some of the most successful lifeforms in Earth’s history.

When they sprung into existence,

they were among the most diverse and sophisticated organisms

Earth had yet seen.

And, as the earliest known animals with complex eyes,

trilobites had a unique perspective on the ancient world.

For almost all of Earth’s history before the rise of the trilobites,

life had mostly consisted of microscopic marine organisms.

But then, scientists think an increase in oxygen allowed multicellular lifeforms

to extract more energy from their food and perform more complex functions.

This then enabled the rise of carnivores,

which in turn spurred a productive arms race,

resulting in what’s known as the Cambrian explosion.

Within about 20 million years,

life had branched out to include most of the animal groups we know today.

Trilobites were an important part of this surge.

All trilobites had three lengthwise lobes,

but building off the same basic components,

they varied greatly.

Species ranged in length from a few millimeters to almost one meter

and some were equipped with intriguing ornaments.

As a result, they filled many distinct niches over the ages.

Many trilobites plowed or burrowed into the sea floor while others swam freely.

Certain species had spines, horns, and even protruding tridents.

And their social behavior was complex:

they came together to search for food, find safety in numbers, migrate and mate.

In fact, their fossilized conga lines represent

some of the first evidence of animal group behaviour.

Like modern arthropods,

trilobites had compound eyes composed of many tiny lenses.

But theirs were made of the mineral calcite,

which also constituted their exoskeletons.

These lenses allowed trilobites to form sharp images

and quickly sense changes in light.

Some had long eye stalks that helped them peep above the muddy sea floor

as they burrowed beneath.

Others had large dragonfly-like eyes

that they probably used to scour the ocean bottom

as they swam upside-down in dim waters.

And some trilobites had pillar-like eyes that may have offered 360 degree views,

along with structures that provided shade from overhead light.

Yet despite their dominance of the seas,

trilobites were vulnerable to environmental changes and predators.

About 444 million years ago, Earth cooled and sea levels dropped,

radically changing some of their habitats.

This was the beginning of what would prove to be the trilobite’s long demise.

Some 20 million years later, fish with jaws began sweeping the sea.

Trilobites developed spiky ornamentation

and neat ways of locking their joints shut.

Many species could curl themselves into balls,

entirely sealing their soft parts inside their hard exoskeletons.

Then, another extinction event shook the world.

Trilobite diversity dwindled, and come about 360 million years ago,

only one of the 10 former trilobite orders remained.

Finally, rapid climate change spurred

the greatest known mass extinction event in Earth's history.

This catastrophic period finished the trilobites off—

along with approximately 96% of all marine species.

But trilobites left a remarkable record behind.

Their calcite exoskeletons made for hardy fossils

that would remain intact for hundreds of millions of years to come.

We’ve found trilobite fossils on every single continent—

many in unlikely environments that were once part of the ocean floor.

Trilobites have no direct descendants,

but their evolutionary cousins are alive and well.

And since arthropods make up over 80% of current animal species,

we might say that, although Earth may no longer be the planet of the trilobites,

their distant relatives still reign supreme.

3

Answer these questions in your own words. Support your answers with evidence from the video.

01According to the video, what made trilobites particularly advanced for their time when they first emerged?
Sample answerThey were significant because they were among the most diverse and sophisticated organisms Earth had seen up to that point. A key factor was being the earliest known animals with complex eyes, which gave them a unique and advanced perspective on their environment.
02How did the environmental shift known as the Cambrian explosion create the conditions for trilobites to thrive?
Sample answerThe video suggests it was a chain reaction. An increase in atmospheric oxygen allowed for more complex, multicellular life. This enabled the rise of carnivores, which in turn sparked a productive 'arms race' of evolution, leading to a rapid diversification of life where sophisticated creatures like trilobites could flourish.
03In what ways did the unique structure of trilobite eyes cater to different lifestyles and environments?
Sample answerTheir eyes were incredibly versatile and adapted to specific niches. For instance, some had long stalks to see above the muddy seafloor while burrowing. Others had large, dragonfly-like eyes for navigating dim waters, and some even had pillar-like eyes that may have offered a 360-degree view for constant vigilance.
04The video outlines a 'long demise' for the trilobites. What series of events and evolutionary pressures culminated in their complete extinction?
Sample answerIt wasn't a single event, but a combination of factors over millions of years. It began with significant climate cooling and a drop in sea levels that altered their habitats. Then, they faced new, formidable predators like jawed fish. After their diversity was already severely reduced by another extinction event, they were finally wiped out by the greatest mass extinction in Earth's history.
4

Key vocabulary for discussing evolution and extinction

Vocabulary
These expressions will help you communicate more precisely and naturally about the topics in the video.
Examples
To stand the test of time — to endure and remain successful or relevant over a long period.
This idiom is useful for emphasizing longevity and resilience, whether for a species, an idea, or a piece of art. It highlights an enduring quality.
To give rise to — to be the cause or origin of something, often a significant new development.
This is a semi-formal phrase common in academic and historical writing. It's a sophisticated alternative to 'to cause' or 'to lead to'. For example: 'Favourable conditions gave rise to an explosion of new species.'
To fill an ecological niche — to find and occupy a specific role or position within an ecosystem for which a species is particularly suited.
While its primary use is in biology, this phrase is often used metaphorically in business or social contexts (e.g., 'The startup filled a niche in the market for sustainable products.').
An evolutionary arms race — a competitive cycle of adaptation between species, where each develops new traits to gain an advantage (e.g., between a predator and its prey).
This concept is central to evolutionary biology. You can use it to describe the co-evolution of species, such as trilobites developing harder shells while predators developed stronger claws.
A long, drawn-out decline — a very slow and gradual process of weakening or reduction that leads to an eventual end.
This phrase emphasizes the prolonged nature of a process. It's often used to talk about the extinction of a species, the fall of an empire, or the failure of an industry.
5

Decide if each statement is true or false. Correct the false ones.

01The fossil record suggests that trilobites were predominantly solitary animals, only congregating for reproductive purposes.
02The video characterizes trilobites as one of history's most successful lifeforms, citing their 270-million-year existence and the evolution of over 20,000 species.
03The durability of their calcite exoskeletons has resulted in a fossil record so extensive that their remains have been discovered on every continent.
04The emergence of jawed fish was the initial major environmental pressure that triggered the long, drawn-out decline of the trilobites.
05Trilobites demonstrated significant physical diversity, with some species burrowing into the seafloor while others were free-swimming, and they varied in size from millimeters to nearly a meter.
6

Complete the sentences with words from the box. One word is extra.

Word bank
01The proliferation of photosynthetic organisms is thought to have given to the Great Oxygenation Event, fundamentally changing Earth's atmosphere.
02The constant competition between predators and prey, where each evolves more sophisticated methods of attack and defense, is a classic example of an evolutionary .
03The fossil record suggests that many dominant species experienced a long, drawn-out before their eventual extinction, often due to gradual climate change.
04After the dinosaurs were wiped out, mammals diversified rapidly to fill every available ecological , from tiny burrowers to large herbivores.
05Despite massive environmental shifts, some ancient lineages like the coelacanth have for hundreds of millions of years, earning them the title of 'living fossils'.
7

Choose the best answer based on what you heard in the video.

01What does the video suggest about the evolutionary legacy of trilobites?
02According to the scientific theory presented in the video, what was the initial trigger for the 'productive arms race' of the Cambrian explosion?
03What was a distinctive feature of the trilobites' compound eyes mentioned in the video?
04The video cites several reasons for the trilobites' long-term success. Which of the following is NOT mentioned as a factor contributing to their prosperity?
8

Trilobites and the Cambrian Explosion

Test your understanding of key vocabulary and concepts from the lesson.

Match each item on the left with the correct item on the right.

A
B
9

Discuss these questions with a partner. Try to use vocabulary from the lesson.

  1. The video describes the Cambrian explosion as an 'evolutionary arms race' that gave rise to incredible diversity. To what extent is intense competition the primary driver of innovation in human society, for example in technology or business? Is this model sustainable, or does it inevitably lead to a long, drawn-out decline for many?
  2. Trilobites were masters at filling countless ecological niches. Thinking about your own country's native species, can you identify one that is particularly well-adapted to its niche? What allows it to thrive, and do you believe it will stand the test of time against modern pressures like climate change and urbanization?
  3. The trilobites' success lasted 270 million years before their extinction. What modern equivalent—be it a technology, a corporation, or a social structure—do you think is currently at its peak but might be heading for a long, drawn-out decline? What factors could give rise to its eventual obsolescence?