We're in the midst of the sixth great extinction right now, with the rise of humans behind the unprecedented rise in the rate at which we're losing species. Some of these species are lost forever, while others are part of de-extinction projects, but all are worth learning about and remembering.

1 of 10 Thylacine Unknown Author / Wikimedia Commons / Public Domain The largest carnivorous marsupial in modern times (standing about 2 feet tall and 6 feet long, including the tail), the thylacine once lived in mainland Australia and New Guinea. By the time of European settlement it was already nearly extinct due to human activity. In Tasmania (which provided the tiger the more common names of Tasmanian tiger or Tasmanian wolf) it lived on, with the last confirmed animal killed in the wild in 1930. The last thylacine in captivity, pictured above, died in 1936. Throughout the 1960s, people suspected that the thylacine may have held on in small pockets, with the final declaration of extinction not occurring until the 1980s. Occasional reports of sightings of the thylacine throughout Australia continue, though none have been substantiated.

2 of 10 Quagga Frederick York / Wikimedia Commons / Public Domain Only one quagga was ever photographed, a female at the London Zoo in 1870. In the wild, the quagga was found in great numbers in South Africa. However, the quagga was hunted to extinction for meat, hides, and to preserve feed for domesticated animals. The last wild quagga was shot and killed in the 1870s, and the last one held in captivity died in August of 1883. A de-extinction project initiated by the organization The Quagga Project in 1987 resulted in the quagga becoming the first extinct animal to have its DNA examined. As a result of this research, the quagga was determined to be a subspecies of the plains zebra, not an entirely separate species, as was previously believed. The first foal in The Quagga Project’s rebreeding efforts was born in 1988, and the group expects that future generations of selective breeding will result in individuals who closely resemble the quagga in color, striping, and coat pattern.

3 of 10 Tarpan Scherer / Wikimedia Commons / Public Domain The tarpan, or Eurasian wild horse, lived in the wild until sometime between 1875 and 1890, with the last wild one killed during an attempt to capture it. The last tarpan in captivity died in 1918. Tarpans stood slightly under five feet tall at the shoulder, with a thick mane, a grullo colored body with dark legs, with dorsal and shoulder stripes. There is some debate about whether the photo above is a genuine tarpan, but the image, from 1884, is claimed to be the only photo of a live tarpan. Attempts were made to bring the tarpan back from extinction, but while the resulting konik horses resemble the tarpan physically, they are not considered to be a genetic match.

4 of 10 Seychelles Giant Tortoise Unknown Author / Wikimedia Commons / Public Domain There's some controversy over whether the Seychelles giant tortoise is extinct altogether or extinct only in the wild. In the 19th century the Seychelles giant tortoise, much like similar tortoise species on other Indian Ocean islands, was hunted to extinction. Prior to being wiped out in the wild by the 1840s, it lived only on the edges of marshes and streams, grazing on vegetation. A study in 2011 indicated a population in captivity of 28 adult tortoises as well as eight adults and 40 juveniles introduced to Cousine Island, which may in fact be Seychelles giant tortoises. A Seychelles tortoise on Saint Helena Island named Jonathan recently made it into the Guinness World Records as the world’s oldest living land mammal—at age 187.

5 of 10 Barbary Lion Sir Alfred Edward Pease / Wikimedia Commons / Public Domain Formerly found from Morocco to Egypt, the Barbary lion (also known as the Atlas lion or Nubian lion) was the largest and heaviest of the lion subspecies. This majestic creature was most likely used in gladiatorial combat in Roman times. Unlike other lions, due to scarcity of food in its habitat, the Barbary lion did not live in prides. The last wild Barbary lion was shot and killed in the Atlas Mountains of Morocco in 1942. However, questions remain about whether some lions held in captivity at zoos or in circuses may be descendants of the Barbary lion, and how best to protect them.

6 of 10 Bali Tiger Oskar Vojnich / Wikimedia Commons / Public Domain The last confirmed Bali tiger was killed in September 1937, with small numbers suspected to have lived on until the 1940s or 1950s. Habitat loss and hunting by humans killed them off. Bali tigers had shorter, darker fur than other tigers. Of the three extinct tiger species (Bali, Caspian, and Javan), Bali tigers were the smallest, closer to the size of leopards or mountain lions.

7 of 10 Caspian Tiger Unknown Author / Wikimedia Commons / Public Domain At the other end of the scale from the Bali tiger, the Caspian tiger was one of the largest cat species to ever exist, only slightly smaller than the massive Siberian tiger. Once living on the shores of the Black and Caspian seas, the Caspian tiger inhabited what is now northern Iran, Afghanistan, the former Soviet republics of Central Asia, and far western China. As population increased in these areas, competition for farmland led to the demise of the Caspian tiger. Beginning in the late 19th century, with the Russian colonization of Turkestan, they began their road to extinction. The tiger became extinct in 1970 when the last of the species was killed in Turkey. Unconfirmed sightings of the Caspian tiger continued through the early 1990s.

8 of 10 Western Black Rhino J. Arnaud / Open Source The plight of the rhinoceros due to poaching has been well documented, and the western black rhinoceros is a graphic example. Once widespread in central west Africa, in 2011 it was declared extinct. Though conservation efforts, beginning in the 1930s, helped the population recover from historic hunting, by the 1980s protection for the species waned and poaching soared. At the start of the 21st century, just 10 individuals remained. They were all killed by 2006. The black rhinoceros, a smaller African rhino, continues to live on, albeit critically endangered, in the eastern and southern parts of Africa.

9 of 10 Golden Toad US Fish & Wildlife Service / Wikimedia Commons / Public Domain In many ways, the golden toad is an iconic species when it comes to extinction. Only described to science in 1966, and once abundant in a 30-square-mile area of the cloud forest above Monteverde, Costa Rica, none of these two-inch-long toads have been sighted since 1989. The reason for its sudden extinction is not known conclusively, but habitat loss and chytrid fungus are likely culprits. Regional weather changes brought about by El Niño conditions are also suspected to have played a role in killing off the last of the golden toads.