Question

The first animals _____. a. arose during the Cambrian b. had an embryo with three tissue layers c. lived in the sea d. had an open circulatory system.

Short answer

The first animals arose during the Cambrian.

Step-by-step solution

Analyzing the Options

Review the descriptions provided in each option to understand what each implies about the early animals. Look for key terms like 'Cambrian,' 'embryo with three tissue layers,' 'lived in the sea,' and 'open circulatory system' as they relate to evolutionary biology.

Understanding Cambrian Period

The Cambrian period is known for the 'Cambrian Explosion,' a time when most major animal phyla appeared. This significant evolutionary event suggests that this could be a period when the first complex animals arose.

Three Tissue Layers in Embryos

Having an embryo with three tissue layers (ectoderm, mesoderm, and endoderm) indicates advanced development but not necessarily the first animals. This feature emerged after the initial appearance of simpler animals.

Life in the Sea

Many scientific sources suggest the earliest animals lived in aquatic environments, particularly the sea, since life is believed to have originated in water.

Open Circulatory System

An open circulatory system is a feature seen in many animals, but not specifically tied to the first appearance of animals. It does not provide information on when these animals first arose.

Conclusion

The first animals arose during the Cambrian, which aligns with the historical significance of the Cambrian Explosion, marking the beginning of complex animal life forms.

Key concepts

Cambrian Explosion

The Cambrian Explosion is often referred to as the "Big Bang" of life due to the sudden increase in the diversity of life forms on Earth. This period, which began around 541 million years ago, saw the emergence of most major animal phyla. It is a key moment in evolutionary biology because it marks when complex animals first appeared. Prior to this explosion, most life forms were simple and lacked the specialized structures seen in organisms today.

During this time, the fossil record shows marked increases in animal complexity, including the development of hard body parts such as shells and exoskeletons. This rapid diversification is thought to have been driven by a variety of factors, including changes in the environment, and perhaps even genetic innovations.

In summary, the Cambrian Explosion laid the foundation for the biodiversity we see in modern organisms.

Tissue Layers in Embryos

Embryos of advanced animals typically develop with three primary tissue layers: ectoderm, mesoderm, and endoderm. This organization is a hallmark of more complex life forms and is known as triploblastic development. Each layer gives rise to different structures and organs in the organism.

• Ectoderm: Develops into the outer covering of the body and the nervous system.
• Mesoderm: Forms the muscles, bones, and circulatory system.
• Endoderm: Becomes the lining of the digestive tract and some internal organs.

The presence of three tissue layers signifies a more advanced level of development compared to earlier life forms, which may have only had one or two layers (diploblastic). However, these structures developed after the first primitive animals, indicating evolutionary advancements.

Aquatic Environments

Aquatic environments played a crucial role in the evolution of life on Earth. The earliest known environments where life is believed to have originated were in the oceans. Water provides a stable medium that buffers temperature changes, supports physical structure, and facilitates nutrient transport.

Living in water has distinct advantages for early life forms, such as easier dispersal of species and reduced effects of gravity on developing organisms. During the Cambrian period, most organisms were marine, thriving in various niches created by the ocean's complex environment. The protection and support offered by water likely contributed to the origins and diversification of life during the Cambrian Explosion.

Overall, aquatic environments provided the perfect conditions for the rise and evolution of some of the first animals.

Circulatory Systems

Circulatory systems are a mechanism by which organisms transport nutrients and gases to and from cells. There are two main types: open and closed systems. The open circulatory system, found in arthropods and some mollusks, pumps blood into a hemocoel with the blood diffusing back to the circulatory system between cells.

In contrast, a closed circulatory system maintains blood within vessels, ensuring a more controlled flow. Evolutionarily, simpler organisms may have first used an open system due to its simplicity and fewer needs for specialized structures.

While earlier life forms might have used basic circulatory systems or none at all, evolution led to more complex systems being developed later. However, the type of circulatory system does not directly inform us about the first appearance of animals but indicates later developments as organisms became more complex.