In this lesson, we will explore the incredible diversity of life on Earth. There are at least 10 million different types of organisms, but biologists have only studied and recorded less than one-third of them. While the variety among these organisms is striking, they also share many fundamental biological characteristics. Organisms are grouped into five main categories: prokaryotes, protists, fungi, plants, and animals. We will examine the differences between these groups, learn how organisms are classified and named, and discuss the importance of biodiversity and the concerns surrounding its preservation.
Biodiversity
Biodiversity, a combination of “bio” and “diversity,” refers to the variety of living organisms within different ecosystems. It measures the range of species present, including both plants (flora) and animals (fauna). The diversity of life in any given area is influenced by factors like climate, altitude, soil, and the presence of other species. Biodiversity is not evenly spread across the planet; it is richest in tropical regions, has many species in temperate areas, and fewer species in polar regions. The biodiversity we see today is the result of 4 billion years of evolution. While the exact origins of life are still not fully understood, evidence suggests that for much of this time, life was limited to bacteria and similar single-celled organisms.
Importance of Biodiversity
Biodiversity is crucial for human survival as it provides food, and many medicines come from biological sources. Additionally, a variety of industrial materials—such as building materials, fibers, dyes, resins, and oils—are derived from plants. Biodiversity also plays a vital role in creating and maintaining ecosystems, helps regulate our atmosphere and water supply, and is essential for recycling nutrients and maintaining fertile soil.
Classification – Aims and Principles
Earth is home to a vast array of organisms, with over 1.5 million known types of animals and more than 0.5 million types of plants, representing just a small fraction of the total species estimated to exist. These organisms vary greatly in complexity, from simple bacteria to complex humans. They inhabit different environments—some live in the sea, others on land, and they have diverse ways of life, including how they obtain food, adapt to their surroundings, and reproduce. With such diversity, it can be challenging to study each organism individually.
To manage this complexity, biologists classify organisms into groups and subgroups using a system called classification. The field of biology that focuses on this is known as taxonomy, while systematics also examines the evolutionary history of organisms. The main goals of classification are to identify similarities and differences among organisms, making them easier to study, and to uncover their evolutionary relationships.
Classification is based on the relationships between organisms, which can be determined by their similarities in characteristics. These similarities indicate that all organisms share a common ancestry at some point in their evolution. For example, sparrows are more closely related to pigeons than to insects, suggesting they have a more recent common ancestor. When classifying organisms, biologists look at both external and internal structures as well as developmental stages. Modern genetics also provides valuable information, as the similarities and differences in DNA help clarify these relationships.
Two-Kingdom Classification System
The two-kingdom classification system is the oldest method of organizing living organisms, dividing them into two kingdoms: Plantae and Animalia. In this system, all organisms that can create their own food from simple inorganic materials, known as autotrophs, are classified in the kingdom Plantae. In contrast, organisms that cannot produce their own food and rely on autotrophs or other sources for nourishment, called heterotrophs, belong to the kingdom Animalia. Notably, this system categorized bacteria, fungi, and algae as part of Plantae.
However, some scientists found this system inadequate because it did not account for certain unicellular organisms, like Euglena, which display both plant-like (having chlorophyll) and animal-like (heterotrophic behavior in the dark and lacking a cell wall) characteristics. Additionally, the two-kingdom system did not distinguish between organisms with prokaryotic and eukaryotic cells.
Three-Kingdom Classification System
In 1866, Ernst Haeckel addressed the first issue by proposing a third kingdom called Protista to include Euglena-like organisms. He also placed bacteria in this kingdom. However, fungi were still classified under Plantae. This system also failed to clarify the difference between prokaryotes (like bacteria) and eukaryotes (like plants and animals). Some biologists disagreed with placing fungi in Plantae, as fungi are not autotrophs; they are a unique form of heterotrophs that absorb their food and have cell walls made of chitin instead of cellulose.
The Five Kingdoms
The five kingdoms of life are categorized based on their characteristics:
- Kingdom Monera: This kingdom includes prokaryotic organisms, which are made up of prokaryotic cells. Most monerans are unicellular, although some can form chains, clusters, or colonies. They are primarily heterotrophic, but some can photosynthesize because they have chlorophyll in their cytoplasm. The main types in this kingdom are bacteria and cyanobacteria.
- Kingdom Protista: This kingdom consists of eukaryotic unicellular and simple multicellular organisms. There are three main types of protists:
- Algae: These can be unicellular, colonial, or simple multicellular. They have cell walls and contain chlorophyll in their chloroplasts, but they do not develop multicellular sex organs or embryos.
- Protozoans: These resemble animals and lack chlorophyll and cell walls.
- Fungi-like Protists: Some protists share characteristics with fungi.
- Kingdom Fungi: This kingdom includes eukaryotic multicellular heterotrophs, such as mushrooms. Most fungi are decomposers; they absorb nutrients by breaking down organic material with digestive enzymes.
- Kingdom Plantae: This kingdom consists of eukaryotic multicellular autotrophs, or plants, which produce their own food through photosynthesis. They have multicellular sex organs and form embryos during their life cycles. This kingdom includes mosses, ferns, and flowering plants.
- Kingdom Animalia: This kingdom is made up of eukaryotic multicellular consumers, or animals, which primarily obtain food by ingesting it and digesting it in specialized cavities. Animals do not have cell walls and are capable of movement
