Class 9 BiologyChapter 12NCERT 12.9 and 12.10
Fossils as evidence, and biodiversity under threat
Rocks remember the life that was. We decide how much of today's life survives.
Think it over
- How can a stone tell you what lived in a place millions of years ago?
- If honeybees vanished from a valley, which other living things would notice first?
- A pond where frogs lay eggs dries up a little earlier every year. What happens to the frogs, and to the birds that eat them, over many years?
In one minute
- A fossil is what is left of a plant or animal from long ago, preserved in layers of rock, sand or mud; stacked up, the layers are a diary of life.
- Dig deeper and life gets simpler; nearer the surface it gets more complex. Indian rocks hold dinosaurs, early humans and ancient plants.
- Four human pressures shrink biodiversity: dirty air, water and soil (pollution), felled forests, taking too much, and a changing climate.
- Species lean on each other, so losing one can drag down those that rely on it.
Fossils as evidence
If life changed over time, there should be a record of it. There is. Plants and animals that died long ago sometimes left preserved remains, fossils, in layers of rock, sand and mud. Layer settles on layer, so the deeper you dig, the further back in time you go.
Dig through time
Youngest layers 1 of 5
Early humans
Near the top: the most recent and most complex forms, such as early humans.
Younger layers 2 of 5
Dinosaurs
Bones and eggs of giant reptiles. India has dinosaur fossils too.
Older layers 3 of 5
Ancient plants
Prints of ancient plants, the kind Birbal Sahni studied to link today's plants with their ancestors.
Older still 4 of 5
Shelled sea animals You may also meet
Simple shelled animals of old seas, preserved in rock.
Oldest layers 5 of 5
Stromatolites
Layered mounds built by cyanobacteria, among the earliest evidence of life. Found in Rajasthan and Madhya Pradesh.
Fossils are the preserved remains of plants and animals in layers of rock, sand and mud. Read from the top down, each layer is older than the one above it.
The pattern across layers is broadly the same everywhere: the older the layer, the simpler the organisms in it; the younger, the more complex. Indian rocks have given up important fossils too: dinosaur bones and eggs, traces of early humans, plants that grew long before flowers. Put in order, the layers become a timeline of life stretching back millions of years.
Quick check
In undisturbed rock layers, the simplest organisms are usually found in:
Show the answer
B. The deepest, oldest layers Older layers lie deeper, and they generally contain simpler forms of life.
Biodiversity under threat
Pull one thread from a woven mat and the weave loosens around it. Ecosystems behave the same way, because every species, tiny or huge, holds something up: plants make food and oxygen, animals move pollen and seeds about, microbes hand nutrients back to the soil. People are pulling four threads right now: we foul air, water and soil (pollution), clear forests (deforestation), take more than nature can replace (overuse of resources) and warm the planet (climate change). When one species goes, the ones that lean on it can dwindle and vanish after it. Play it through with the Pakke hornbills.
Hornbills nest in holes in big, old trees and eat their fruits. Seeds they drop grow into new trees.
- 1A connected forest
Hornbills nest in holes in big, old trees and eat their fruits. Seeds they drop grow into new trees.
- 2Old trees are cut
Deforestation removes the large trees, and with them the nest cavities.
- 3Hornbills decline
With nowhere to nest and less fruit, fewer hornbills breed. Their numbers fall.
- 4The loss spreads
Fewer seeds are carried, so fewer fruit trees grow back. When one species goes, others that depend on it may follow.
- 5Conservation
Protecting old trees and whole habitats, as in tiger reserves, lets the web rebuild. Classifying species tells us what is at risk.
Classification is part of the answer. You cannot protect a species you have not identified, and naming new ones shows what a place holds. When a new organism is found, scientists compare it with known species; its name may mark where it was found or honour a scientist. Meet a frog named after hills.
Nasikabatrachus sahyadrensis
- Name The species part, sahyadrensis, honours the Sahyadri hills, the Western Ghats.
- Lifestyle Stays buried in soil nearly all year. The monsoon rains bring it up to mate; then it digs back down.
- Ancient Fossils show it belongs to a very old family of frogs.
- 2003 Discovered in Kerala in 2003, it helped scientists understand ancient amphibian groups and showed why the Western Ghats need protecting.
Dry season: the frog is buried deep in the soil.
Quick check
Which human activity listed below does NOT directly reduce biodiversity?
Show the answer
C. Planting native trees in a cleared area Planting native trees restores habitat. The others all reduce biodiversity.